Short answer: Bad luck.

Long answer: If Player X tries to get on the German side (Leaving the game until they get a German plane, then accepting it), then the spot that Player X occupies on the server (Spot X) is either empty (when he's between quits), or occupying an Axis slot. Therefore, Allied spots are more readily available, as they are in less demand.

That's all assuming that there is a tendency to even teams out somewhat. If it is truly a random selection with replacement (Meaning 50% chance of either side, regardless of what others are playing as), then this doesn't apply, and the short answer is 100% true.