Is my outlet grounded?
Metal junction boxes are typically serviced by metallic conduit that is grounded in the panel. This is particularly common in older homes that have been updated to whatever the current code was when it was rewired. Homes 100 years ago didn't require grounded outlets so when they were brought up to date it was the best way to provide proper grounding. Although a receptacle will usually ground through the attachment ears to the metal box you should still attach a ground wire to the green screw on each recptacle and run it to a grounding screw on the box.
One suggestion for the future, instead of a receptacle tester, which can be squirrely at times, you should invest in a multimeter that will give you more precise information.