TWO prong outlet indicating 'correct' / not indicating an 'open ground'??

[Ryan Murphy]

Here is a photo of a TWO prong outlet indicating 'correct' / not indicating an 'open ground'. (Using a 2-3 prong adapter.) How can this be possible? p.s. I'm pretty sure the tab on the adapter was not touching the plate cover screw.
33547
Thanks

*Had this posted under 'building envelope' section by mistake

[Jerry Peck]

Main discussion is over here on this thread: http://www.inspectionnews.net/home_inspection/building-envelope-home-inspection-and-commercial-inspection/48894-2-prong-outlet-tester-indicating-grounded.html

[Gunnar Alquist]

Here is a photo of a TWO prong outlet indicating 'correct' / not indicating an 'open ground'. (Using a 2-3 prong adapter.) How can this be possible?
33547
Thanks

*Had this posted under 'building envelope' section by mistake

Buy a new and better tester.

[Ryan Murphy]

I'm thinking this may have been on a GFCI breaker (in the panel). That would give this result, no?

Also, aside from the amount of voltage, what more information would a wiggy/voltage tester have been able to provide me over the receptacle tester? I'm believe that I need to get one, I'm just trying to understand what more it will give me and in what situations I should use it.

Thanks

[Jerry Peck]

I'm thinking this may have been on a GFCI breaker (in the panel). That would give this result, no?

No, because there is still no "ground path" for the tester to read, other than the adapter's metal tab touching a grounded metal screw in the cover.