I've just reading back on some engine tests where the tester (Peter Chinn) gives figures for both geometric and trapped compressions to give an idea why I was surprised by your figures. Not having a shot at you here, I'm just trying to be helpful in case something got overlooked. For instance, when you used the syringe did you fill right to the outer edge of the squish band? Most of them are tapered so take quite a bit extra to fill other than just the bowl itself. Anyway, here are some of the figures I found with geometric first then trapped and the final figure is when the exhaust closes ABDC. The closing figure is important because the higher timed the lower the trapped compression will be compared to geometric.

Super Tigre S61H...11__8__74
Super Tigre S75....10.8__7.6__75
Enya 60X....11__8__72.5
Fox 78...12.5__9.2__72.5
OS Max-40PS...12__7__93

The first 4 engines are all very much the same with a difference of about 3 between their two types of compressions and all with similar exhaust timings. The interesting one is the OS which in this case happens to be an all out racing engine with very high timing giving that difference of 5 between their compressions. Also it's designed to run on zero nitro which would make it seem that the 12:1 compression is lower than it could be for methanol but it's also designed to be run only with a full length tuned pipe which supercharges the cylinder so gives a much higher effective compression.

I guess I should add that all the above engines are around 30 years old and compressions on more modern engines have been lowered a bit since then being more like 9:1 (geometric ) or thereabouts.