Hi Terrywoodenpic,
differences you are talking about are around 1-2mm. while i have noticed more than 10 mm.
have a look at the well known database (i have checked some points and the results were the same)
http://wiki.panotools.org/Entrance_Pupil_Database
17-40 (17) L2=71
18-55 (1

L2=47
10-22 (10) L2=66
L2 is the only variable value when change the lens. Distance between lense base and head mounting (L1) is a constant. L1+L2=NPP
So the difference between lens NPP's must be the same regardless panohead used.
Now let's see at the fanotec's data (30D NN5):
17-40 (17) B=106
18-55 (1

B=88
10-22 (10) B=94
66-47=19
94-88=6
13mm!!!