Overlapped shock ground seems to apply even if immun to it

I have 100% reduced effect of shock thanks to my spec / clusters, but I noticed that the shock ground shows some different number sometime.

It seems to be when there are overlapped shocked zones.
When I enter the first one I got a shock debuff with 0%.



Then when I step on an overlapped zone I got the debuff with 50% shown.



Edit :
I tried with chilled ground and it does the same.
But it seems to be only a tooltip issue, I did not get the slow from the 30% chill.