It means you have two deteriorating iron pieces close together. The "halo" affect of iron rusting underground...especially in wet ground, gives off an electromagnetic signal very similar to silver on the outer edge of the "halo." This becomes more evident between two pieces of iron spaced at the right interval. It has been my experience that at the edge of a 3-6 inch deep piece of iron (especially two pieces about 6 inches apart, and 3-4 inches deep) I can get a solid, repeatable penny/dime signal at several angles. That's not just with the MXT, but also with a higher end Bounty Hunter and an ACE 250.
The solution to that problem is, if you attempt to pinpoint and get the results in question, turn 90 degrees and scan again, and I almost bet you will get two iron signals...Also, one of my preferred techniques is to take my knife, find the space where I am getting a "questionable" penny/dime signal, find exactly where it seems to be coming from, and stab just shy of the indicated depth and wiggle the knife around a bit. Rescan and about 95% of the time, the signal disappears, or turns to iron. If it gets stronger, I dig and get that darn coin out of there!
On a side note, I would try and ask questions like this in the SE forum...but the MODs will probably move you anyway.
Either way...MAN I LOVE MY MXT!!! Happy hunting!
~Tom