I was helping my friend who purchased three packages this year. One of the hives has many, many more bees than the other two. So many that there is a large clump of them around the entrance (top entrance). It almost looks like they are getting ready to swarm (they are not, just looks that way).
The packages were all the same size, roughly the same number of bees. We installed the packages on May 5th, so it's too early for the new bees. The only thing I can speculate is that the queen pheromone is much stronger in the hive with the excess bees and she has attracted bees from the other hives. The hives are positioned side by side, north to south, with the entrances facing east.
Any ideas why there are so many more bees in that hive, so much so that they are congregating around the entrance?
http://www.bushfarms.com/beesfaqs.htm#bearding (http://www.bushfarms.com/beesfaqs.htm#bearding)
Could be bearding or drift as you mentioned. If they are jam packed & you have any kind of flow on consider adding (well, your friend) another box. You should see what our hives look like late summer down here. :-D
...JP