Layer averaging (winds)

[Jason E Patla]

Aloha Group,
I'm trying to determine the average winds vectors with files of multi-layered U (u) and V (v.2) wind components.  I am unfortunately getting mixed results using the AVE and MEAN functions.
For example, when trying to display the layer average for 200mb to 500mb, the two following commands yield different results (only the order of the levels are switched):

d ave(u,lev=200,lev=500);ave(v.2,lev=200,lev=500)

and

d ave(u,lev=500,lev=200);ave(v.2,lev=500,lev=200)

I would have guess the average is the average regardless from which layer you start or finish.  I get the same differing output using the MEAN command.  Any suggestions?  FYI, the levels in between are 200, 250, 300, 400 and 500mb.
Thanks in advance for your help.
Jason