Well, I know this, my 83 300D with 387K and change on the clock always pegs the oil pressure gauge at max on cold start. It usually stays at max during idle until the engine is warmed up, then it drops to about 1.75 to 2 bar at warm idle.

If you are really hung up about the gauge, you could always either swap in another mechanical type gauge as a test, or hook up an oil pressure gauge to the port on the engine and see what is going on.

It is possible that the gauge was sticking internally for some reason and would not go beyond halfway, and when it was removed it was jostled enough to free up the inner workings. A stretch yes, but still possible.

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2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)