This is school dependent and the cut-offs can be higher than a P but change from year to year (for instance, two years ago at Queen's University you needed a writing score of R in order to get through to the interview stage, last year it appeared to have dropped to Q). It is thought that the hard cutoffs are used arbitrarily to cut the candidate pool down to a more manageable size without putting too much work into it and given that there are many more qualified applicants than there are seats available, they can make these cutoffs quite high. Another thought is that these universities that use high cutoffs for the writing sample and/or verbal are looking for qualities beyond just being able to memorize the different stages of the Kreb's cycle or how to calculate resistance in a parallel and serial circuit. I guess the American schools just look for these qualities in other areas, such as ECs or having completed a degree in the humanities, rather than the ability to write a paragraph with many multi-syllable words in a very short amount of time.