GF 500mm telephoto not as sharp as expected

[EdCampbell]

Hello all you GFX system gurus, I've been experimenting with my new 500mm telephoto on my GFX100S for a couple of weeks now.  Even when I'm extremely careful, bring along a monopod, shoot at 1/500 sec, etc., the results are not quite there for me sharpness-wise.  So today I gave the lens a controlled indoor test, ISO 100, f/8, tripod mounted, 100% flash exposure, using the tightly-knotted back of an oriental rug as my target.  I repeated the test with my GF 45-100mm lens, matching the same portion of the rug.  The results for the GF 45-100mm lens were very noticeably superior, even though using Fuji's MTF charts for each lens the results should have been nearly identical.  Is there any reason I should I expect this discrepancy?  If not I'll try to return the lens.  Thanks!