Basically, it comes down to the fact that the countries have different laws, and different specific rules about things, and thus a car seat that's approved for sale in one jurisdiction isn't necessarily in the other. It's not that one's explicitly safer than the other; it's just that they are different, and so manufacturers have different versions for each country. The ones approved in any one country aren't technically available in others. However, Graco does have a Canadian version of the Modes line, here. They just can't sell the specific US-apprived model in Canada (and you may not buy it in the US and import it to Canada); similarly you could not do the reverse, either.
See this article from a Canadian insurance company, for example, which mentions that US car seats are explicitly disallowed:
- It is illegal to use a car seat purchased in the United States
Child car seats can run to a few hundred dollars, so it can be tempting to look for a good deal south of the border.
However, safety rules are not the same in Canada and the United States, so a car seat bought in the US may not meet Canadian standards. The SAAQ categorically states that using a car seat bought in the US is against the law.
Transport Canada, on the other hand, says that it is not illegal provided that . Basically, if a model complies with Canadian as well as US standards and carries the National Safety Mark, you may be allowed to import and use it.
Of course, it's very difficult to check all this if you buy the car seat online and have it shipped to you.
The Transport Canada page that site references explains in more detail what the issues are. It gets down to the specific problem here:
Child, infant, or booster seats purchased outside Canada, including those purchased online from non-Canadian vendors, may not comply with Canada’s Motor Vehicle Restraint Systems and Booster Seat Safety Regulations and the applicable Canada Motor Vehicle Safety Standards (CMVSS), and thus would not carry the National Safety Mark. Every country has its respective child seat standards. Canadian regulations are rigorous and differ from those in other countries, as explained here.
And then you can read the Canadian regulations here. The US equivalent is here; they're pretty similar to my untrained eye, but not identical - hence the differences.
One of the major differences seems to be related to Compression/Deflection testing, which refers to how much energy needs to be absorbed by the carseat in anywhere an impact could occur (such as the head hitting the seat back). Canadian seats might have more foam in certain places (such as the headrest) than US seats as they focus more on that. Whether that makes the seats safer in any significant manner, I couldn't say. The specific regulation I believe is this one:
Surface contactable by head
213 Every surface of a child restraint system that is contactable by the head of an anthropomorphic test device positioned in the restraint system in accordance with subsection 4.4.2 or 4.5.2 of Test Method 213 must be covered with slow-recovery, energy-absorbing material that, when tested in accordance with section 6 of Test Method 213, has
(a) a resistance of not less than 4 kPa but not more than 70 kPa at 25% of compression-deflection resistance;
(b) a thickness of not less than 12 mm, if the material has a resistance of not less than 12 kPa but not more than 70 kPa at 25% of compression-deflection resistance; and
(c) a thickness of not less than 19 mm, if the material has a resistance of not less than 4 kPa but less than 12 kPa at 25% of compression-deflection resistance.
The US regulation (and the Canadian one also) does mention the allowable g forces on the head (60g for any force felt for > 3ms) during testing, and seem to be identical (perhaps intentionally so).
I found [CalSpan's explanation particularly interesting]; CalSpan is a company that among other things carries out crash tests for manufacturers. Here's what it mentions:
Federal Motor Vehicle Safety Standard No. 213 (FMVSS 213). This U.S. standard requires CSRs to pass a 30-mph frontal sled test, which simulates a crash. This standard applies to passenger cars, multipurpose passenger vehicles, trucks, buses and aircraft.
Canadian Motor Vehicle Safety Standard No. 213 (CMVSS 213). This Canadian standard is very similar to the U.S. standard. One of the differences is that the CMVSS 213 has a requirement for compression/deflection, which affects the energy management in terms of force on the head.
European Test Standard for Child Restraints ECE R 44. This European Standard requires all CSRs be certified and have an ECE R44/04 certification label to indicate they comply with standard safety requirements.
NHTSA Side Impact. In this test, which simulates a side-impact vehicle crash, CSRs must demonstrate they can safely restrain a child by preventing harmful head contact with an intruding vehicle door and reduce the crash forces transmitted to the child’s head and chest. This includes using the Q3S crash test dummy.