oil pressure reading?

Just wondering....last year I installed an AutoMeter short sweep electrical oil press gauge and until today it always reads 38 - 40psi at idle or WOT even with me shifting over 5000rpm in 80+ degrees weather and when the weather is below 60 degrees she's a little higher at 43 ... Today during a 20mile cruise @ 3000 rpm (4.10 rear) I noticed my oil pressure was 32. I know the rule of thumb is 10lb for every 1000rpms but why did it start reading correctly now? When I went WOT the pressure went to 40lbs but no higher @ 5500rpms. Shouldn't my gauge read 55 shifting @ 5500rpms? Is my gauge F'ed up? I don't have a mechanic gauge handy to test her just 'fishing' for some insight. Its a 273 commando mild mods, big cam (.515/..525) and I run VR1 20W-50.

ZooKypr