1957-D Washington Quarter - Strike/Die Error On Motto?

I have never seen an error quite like this one, if indeed it is an error, and would appreciate some help identifying what might have caused it.

It is in the motto of a 1957-D quarter. The coins seems uncirculated, original surface, some of the flow lines are still visible, especially in the field. The motto is really thin on the left most letters,IN and T in the second line. It seems discrete, that is rather than the letters gradually getting thicker the further right the motto goes, it seems like there is a definite change between the IN/T and the rest of the motto. I can't tell if there is this change in height on the letters in LIBERTY The UNIT in UNITED on the back (same side) seem to have the same effect.

The letters don't show obvious wear and the edges seem as sharp as the other letter's.

Would this be a case where the die has simply worn down on that area? What would cause the die to be worn there more than other places?

I am really curious. Any insight would be appreciated.

Attached is a photo of the coin