Why do cover cutoffs vary in PvE?

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Why is it, depending on the character, the amount of covers given out in PvE varies? I know 4*s are different, and Thor doesn't really apply to this, but why is Gamora only being given out in multiples to the top 20? It irks me every time I see this, and it's a frequent occurrence, just taking the cover out of the top 50 reward for no apparent reason.

Thick As Thieves gave out Rocket & Groot to top 150, 2 covers at 50, 3 covers at 10. Now, Gamora is being handed out at 2 covers to top 20 only. ****?