In regards to the lower LSAT score averages, you have to understand that on the whole, the Canadian education system does not use standardized tests for admissions purposes. On almost all levels, admission is based on your academic performance at the previous level.
We do not have SATs or any tests for admission to undergrad. (though a few provinces have recently brought in some tests required for High School graduation (ex. Ontario's grade 10 literacy test)). Your admission to an undergrad program is based on your HS grades, and possibly soft factors depending on the school/program. Also, most Masters programs in Canada do not require GMATs or GRE's or whatever they're called.
Canadians are at a bit of a disadvantage on these tests, because for most of them writing the LSAT, it's the first and only standardized test they've ever written. We're not used to the prep, format, and grading that is used on it.. Of course, this doesn't explain everything, but I'd imagine it translates into about a 5 point drop in average scores. Law School's know this, and take it into account.
Also, don't be surprised to see the LSAT requirement for Canadian schools disappear in the coming years. There is a review happening right now over the finger printing involved -- because under the Patriot Act, the US Gov't can demand a copy of your fingerprints & personal info from LSAC - and LSAC can/will not tell you about it. Thus, Canadian law schools are essentially requiring you to submit your fingerprints and personal info to a foreign government as a requirement of admission - which is a violation of personal freedoms and civil/privacy rights. I'm not completely sure of where this whole issue stands right now, but I do know that Canadian schools are looking into alternatives to the test.