Discrepancy between GIA Screening Device and Multi Tester

I bought a diamond accompanied by a GIA grading report, but then encountered an inconsistency during testing.

When the stone was tested using several Presidium Multi Tester III units, the results consistently indicated moissanite; however, when I brought the stone to a lab and tested it using a GIA iD100 Diamond Screening Device, the result indicated that the stone is diamond.

What are the posible reasons forthis discrepancy between the handheld tester and the GIA screening device?