Ian, the target ID's on ANY detector, are ... as Jeff said ........ only telling you conductivity. NOT actual composition. They don't tell you what a target *is*, aside from it's conductivity. And while a penny or dime or quarter may have a distinctive signature (TID), yet so too can other objects share those same ID's. And don't forget, size plays into the conductivity of an item.
For example: take a small snippet out of an aluminum (or just take the tab), and wave it in front of your machine. Gives a foil or tab or whatever signal. Right? Ok now take the entire aluminum can and wave it. Reads penny or quarter or half or whatever. Right? BUT WAIT! The composition of each object NEVER CHANGED. In each case, it's still aluminum. So what changed? The size did. See? However, with practice, anyone can tell the size difference, just by audio, of the difference between a tab or finger-nail sized item, versus an entire-can sized item
And so too should it have been for the big iron object you found: While it may have read 80 (which is commensurate perhaps with a higher conductor coin), yet I'll bet there should have been audio clues. Not sure about your particular machine, as I'm not too familiar with the AT-pro. But on most machines the audio difference between a 2" square piece of rusted iron should have a difference, audio-wise (not TID wise) than a clean coin-signal. Perhaps the AT pro doesn't have such an expanded range of tones to learn. Some machines are just "beep or no beep" machines, with little tell-tale audio clues. You can switch to all-metal pinpoint, and sometimes the relative comparison can be learned as well. Or get a better machine. Because, for example, I know that with the explorer, even there would be a sound difference. And with various whites machines like the XLT, 6000 di pro, etc... the trick is to momentarily speed up the swing. Iron objects will "break up" and cease to be "clean". But if they were a coin or conductor, they "get better" with faster swings.
All in all, I'd say you must've watched the Whites commercial and fallen for the hype of "tells you what's in the ground" . Doh!