I have seen at an antique shop where all 3 legs were ground down. I asked the shop owner what happened to the legs, he replied that one had a piece broken off so he ground them all to the same height. He did not do a very good job and it still sits in his shop.
It can be expected to find legs worn, or one with a leg broken. Yes, it takes from its value, but not as bad as grinding them all down.
Yours might just have normal wear, like Doug stated from friction dragging it across firebrick.
Something to think about the fact that the some legs could have been eroded by the fire itself.
Not all hearths were of brick, a great many early ones were field stone granite.