Having a random number of items in the creep waves is fine.

#IF# It's the same random generated number on every player's board.

Getting different numbers of drops on that difficult late game raptors wave because you couldn't clear them all? Fair. Getting different numbers of drops on the first three minion waves that everyone 100% clears? Not fair.

Just make the game decide how many drops every player's waves will have first, then hand them out to random creeps. This way failing to clear every creep can result in different numbers of items, but everyone who perfectly clears the round gets roughly the same level of reward (what items drop are still random on a per-drop basis).