As promised, I found information on a NASA website concerning lightning rates. They got a satellite that measures this stuff. There appears to be a strong correlation between upward convection and lightning flashes. And climatic activity over the oceans is markedly lacking in this upward convection. Hence I am not crazy, lightning really does flash more rarely on Guam than on the mainland.
Why the sudden upsurge in lightning around Guam? That’s a mystery to me still.