Something is causing the iLIGO HAM1 Optical Table to be too low.

First I measured the weights of the masses I'm using for the payload.  A drawing error has me 11.254lbs heavy; I don't think this is enough to make a large effect.  D080001 & 2, HAM Trim Mass Large & Small have their weights noted in lbs; looks to me they should have been Kg.  So D080001 is noted to be 1.14lb, I measured 1.106kg; D080002 is noted to be 0.26lbs, I measured 0.25kg.  I'll put redlines in the DCC.
The Spacer D1200530 (between Support Tubes and Support Table) measured as drawn as I remember Mitchell confirming when we received these.
I measured the Gap between the two Leg Elements (large masses in the isolation stack) in a few places to measure the compressed height of the Spring.  This value is 1.86+-0.01".  I did not measure an uncompressed Spring.  T1000310 (PeterF's HAM1 Isolation Stack doc) notes a 2.03" tall Spring with 0.14" of compression.  So about 3/4mm more compression (still early days too) x 3 for the three layers of Springs in the system.  So, this would put us 2.5mm low...OK not enough there.
Sam Barnum of MIT reports to me the compressed Spring height in the model (used to determine the Spacer height?) is 1.93" putting us 5.25mm (over 3 layers) lower than expected....
So I don't see enough from these listed issues to get us to 15mm, maybe 6mm low...
Other possibilities are the leg elements and Optical Table are heavier than modeled...maybe I can put some numbers on that soon.