I've been running a bit of Last Carnival lately, and I've noticed a very clear difference in drop rate of Grutin Cube. After about ~50 runs, I ended up with half as many Cubes (of every color) as the rest of my team mates. I have tried on different days / time / with different people / with or without Gronco buff, but the result is pretty consistent - about half as many cubes. I'm fairly certain this is not just me being "unlucky", given how frequently those Cubes should be dropping, and given the number of attempts I have tried.
So, my question is: Besides + drop chance buff from Gold VIP and Gronco buff, what else is affecting drop rate?