like Tec said it comes from to the genetics of the bees. Italians produce very little Caucasian produce a lot more. Al though I don't discourage propolis production there are occasions when the hive gets carried away with collecting propolis well beyond the needs of the hive.
I had a hive once that in addition to continuously closing off the bottom and top entrance, they would also pile it up on top of the ends of the top bars. This was happening in the middle of the hot summer the propolis was running down the face of the combs due to a lack circulation. I would scrape it clean and with in a week it they would have it plugged again.