Discrepancy between @ave and @sbx?

["Neven S. Fuckar" <nevensf@xxxxxxxxxxxxxx>]

Hello,
I am 1st year grad student and new to the realm of Ferret, but I noticed
one point that could be possible glitch or maybe (not surprisingly) I'm
still not appropriately familiarized with Ferret (V 5.22 for DEC Unix). 
While examining properties along the isopycnals I noticed that two
averaging procedures that suppose to be the same yield different
results. For example:

fill tiso[l=140@sbx:60]
contour/over tiso[l=110:170@ave]

@ave approache contoures lines over areas where @sbx leaves blank space.   
I would appreciate comment or explanation of this issue.
Thank you in advance for your understanding and time.
All the best,
Neven

-------------------------------
Neven Stjepan Fuckar
Texas A&M University
Department of Oceanography
College Station
TX 77843-3146, USA
phone: 1-979-845-3238
-------------------------------