Holding the throttle wide open might give it a handful more PSI. Cold to warm, same thing...warm should give you a higher reading than cold, when the engine warms up, the piston and rings expand...that's why when an engine is low on compression and worn out, it's harder to start cold, but once warmed it usually starts much easier...
Either something isn't right, or the gauge is bad.
Is the gauge thread depth as long as the spark plugs? That could falter a few PSI.
That bike should be 170 to 190 PSI roughly....