Sounds strange to me. AFAIK it's count=2, duke=3, king=4, emperor=5, straight up. Only thing I can think which would drop it (other than stewardship stats changes of course) would be if you had Gavelkind succession and changed it to something else. Gavelkind gives a 1.3 times bonus (I think) on your stewardship based demense allowance.
Mouse over your demense limit and check out the breakdown in the tooltip. If you have a savegame you could go back and compare.
D'oh, thank you!!
I forgot I changed to Elective to avoid it getting in the hands of a bad branch of my family.