thats exactly what i had thought, that you need to get crit above a certain threshold to enter a new 'tier' of crit, so to speak. An attack will either crit or it won't; the chances of it doing so would be greater with each tier. but each tier operates a range, say, between 480-510 (made up numbers, but you get my point). So having crit at 505 doesn't necessarily mean you'll do greater dps then if you had crit at 485 (using those made up numbers).

Whereas det, str and wd all seem to scale in a linear fashion.