Plug a drop light into GFCI - the light lights up! Plug drop light into house AC - the light lights up.
Plug an extension cord into GFCI and test with polarity tester - all is well with gnd, hot and neutral.
Plug (same) drop light into above extension cord and the GFCI "blows" (i.e. cuts the circuit breaker).
The resistance of the extension cord only looks like a bigger drop light bulb. So resistance is not the problem.
Plug a "cheater" into extension cord (above) and plug drop light into the cheater (i.e. ground wire is not connected) making sure that there are no possible ground contact points - GFCI "blows" [see above] ! So the grounding is not a problem.
When the GFCI compares the AC current in with the AV current out it is obviously comparing similar phases (a.k.a. cycles). Clearly any phase shift due to capacitance in the extension cord COULD cause what we see.
- Rick