I have ddone lots of reading here and simply do not understand the tank pressure...

I have low water pressure..I need help understanding why someone needs to drain tank to test pressure...My system is set up for 30/50 on the switch and I noticed that with a full tank, gauge at 50, the pressure in tank is 47psi, and when tank is almost empty down to just around cut in at 30psi on switch tank has 27psi pressure....There seems to be this direct correlation with pressure on tank and reading on gauge? Or am I missing something? When I adjust switch to come on at 40 and go off at 60, correlation still exists, tank readings are now 37 near cut in and 57 at cut off....so why does someone have to completely empty tank to test there pressures? can someone not simply adjust switch and not touch pressures...???

Reason I ask is that is what I did, just adjusted switch to bring setting from 30/50 to 40/60....never had the time last night to drain tank fully to check pressure in tank....Is it really necessary?

Pressure at facuets seems totally fine now.....What are your suggestions if any???