How crits (or any RNG attack modifier, really) works is that there is a priority. Say for instance you're playing PA, and you attack a creep. The game checks to see if that attack triggers a crit. If it does, crit! yay!
Now, if you had multiple "proc chance" items, it would check one after the other, to see if anything happens. So first the PA crit, then daedelus, then maelstrom, for example, although of course not all can happen at once. The game checks first one, then the other, effectively increasing the chances of a crit.
Back to the question, daedelus is good on PA because it gives her more crits, but not necessarily of the same modifier as her ult.