They aren't entirely the same game. Kills can never win a Zone Control match, so it more effectively demonstrates which team is capable of controlling more of the map than another team.

And Iron Banner tends to get more coordinated teams playing together than normal Crucible, so the results reflect, for better or worse, your teamwork.

Each of these small details is enough to make a significant impact in the data, especially given that your rating is relative to everyone else's ratings.

But I would agree, that a smaller sample size can be beneficial if you played particularly well. You also need to consider the learning curve for each game type. You played Control first and thus brought all the lessons of Control into Iron Banner, and having played both of those you brought all those lessons into Zone Control; thus, Zone Control benefits more greatly from your experience than the others. This same anomaly is why my most played character has a lower overall K/D than my least played, because of all the hours I put into that character learning and failing, where with my lower played characters, I've already benefitted from all the failures on my other character.