I do believe the pressure drop theory as on a valve that imply unscrews there's nothing to stop air escaping as you unscrew.
There is, the pump head has a seal. As you unscrew the valve on the shock is closed before the seal on the pump is broken. So in effect you have 100psi is the fork, sealed, and 100psi in the pump which will be released when you finally unscrew enough to break the pump seal.
Im the OP case it could simply be the pump is old and the o-ring seal is not holding pressure like it should.
1)apply gauge to fork, reads 100psi.
2)take gauge off
3) put gauge back on reads 100psi
4) apply pump to fork, reads 100 psi
5) take pump off fork
6) apply gauge to fork reads 80psi
I dont understand how step 4 gets you 100psi, as has been said when you attach the pump it need to be pressurised by the fork, this is the step where many think they 'lose air'. So it should be showing 80psi when you attach, hence why i thought your pump was reading 20 above your guage. Im not doubting you, i just dont understand how.
unfortunately its not 20 every time, varies from 10-30.
It does seem to be to do with how fast I unscrew, but its still not a good system, no matter how fast I do it I still lose a lot of air.
I think you might be best looking at a marzocchi pump, might be worth looking at a lower pressure pump rather than a 300psi shock pump as they can be less accurate outside the nominal working range.
Possibly change the o-ring in the current pump first.