I noticed that my PC was quite frequently only gaining 1 xp from killing baddies. I gathered a small sample of data:

My character:
Mountain Dwarf
Fighter 3

Killing things on level 2 of a dungeon:
Key: Monster (xp gained for different kills of these monsters during this test)

giant rat (11, 12, 12, 15)
orcish warrior (1, 6, 1, 1)
goblin scavenger (1, 4, 7, 11)
goblin warrior (1, 1)
human ruffian (8)

Then on level 3 of a dungeon:

giant rat (13, 12, 13, 7)
orcish warrior (7, 1, 3, 10, 1)
goblin scavenger (1, 1)
goblin warrior (1, 1)

Not a very large sample, but it seems enough to cause suspicion that there are some unintended inconsistencies...