You can retake the test as many times as you want. Some schools average them, others will take the highest. The whole point of creating the test(s) in the first place were to deal with variability of GPA's between schools, so it's a bit useless to say "just adjust GPA's between schools instead".

In the US now we have federal school testing to try and normalize schools better. But before that many schools were graduate students that were illiterate (actually some still are, but I hope it's gotten better..). The US is 10x the population of Canada so image the difficulty for a liberal arts school in the northeast looking at a student from some rural 500-person school they've never heard of from the southwest.