You know PPM is adjusted for weapon speed, right? That's why it's PPM. A 1 PPM weapon is set to proc once per minute if you just autoattack with it, which means (say) with a 3.0 speed weapon it'd have a proc chance of 5%... but a 2.5% proc chance on a 1.5 speed weapon, so on average you'd only get one proc in a minute with both.
This gets heavily skewed since instant attacks (Whirlwind, Bloodthirst, Hamstring et al) _also_ use that proc chance whenever they attack... so while you'd get the same number of procs when autoattacking, you'll get a hell of a lot more procs off instant attacks when it's a 5% chance per instant than a 2.5% chance per instant.