I think it has to do with which sequence is first in the NIF/KF.
I dcan't understand which could cause this. Any idea?
I find that hitting the print screen button fixes it.
Just tried, you are correct
However, this is not only a preview of the cs, but it is also how these creatures are placed in game using the commands placeatPc and Resurrect. Here is an example with the Bonelord.
http://img28.imageshack.us/img28/9386/screenshot416.jpg