Inconsistency with Cull

I was trying Cull out on some of my ADC's and I noticed an inconsistency with it. It seemed that when my support popped his relic shield stacks it would count towards my cull, some of the time. It would not count toward my cull some of the time either. I'm not sure what causes this but I know I was playing, my support popping relic shield stacks and I had farm equal to my cull stacks (well, lack thereof). I continued farming and my support popping stacks then I was offset by 1 or 2, but my partner had popped some 10 or 11 stacks, so there is an inconsistency somewhere.

I suspect it has to do with popping relic shield on cannon minions, but I haven't isolated the difference here.