Zed's passive

I'm just wondering if there is something wrong with 's passive. It appears to not be proccing on champions until they are around 35% health. Tooltip says it should be at <50% health, and the passive procs on minions just fine. Am i missing something here?

@CertainlyT or anyone else who can shed light on the subject :)

edit: by "just fine" i mean that the passive procs on minions at <50% health as stated in tooltip