With a combined 11% chance (3-3-3-2) it procs only about 5% after killing a foe. Limited sample of 250 but this adds more evidence to a general observation.
So either it's bugged or "Multiple copies of this enchantment increase the chance of spawning the Wisp" does not mean the percentages simply add up.
It could be a bug related to same levels of this enchantment not stacking. Silvery enchantments showed a similiar behaviour in the first weeks of LIVE. Several Rank 5's wouldn't stack while R4+R5+R6 was working fine. So it could be that this is a classic not-stacking kind of bug and that your actual chance right now indeed is 3% + 2% = 5 %.
Comments
But since there is no official word: Still an issue under NW.5.20130923a.7