Hi Solomon,
What people are getting at:
The signature of your sword doesn't match those of authenticated examples in our references. What's more, the characters are cut clumsily (the gap between Kuni & Hiro is especially bothersome to me). While it is possible that the signature is by some unrecorded, later smith, it is more likely that the signature is a gimei; it's a forgery. Forged signatures are very common with Nihonto, and Hizen Kunihiro is well enough known that forgeries are to be expected.
The horimono (artwork/carving) is very poorly done, nothing any self respecting smith would allow on his blade. Sometimes horimono is put on a blade decades or even centuries after it is made to cover/remove a defect that has surfaced due to repeated polishing; this is a possibility here. It is also possible that the horimono and questionable signature were done at the same time, not terribly long ago, to make an otherwise uninteresting sword more desirable to the unsuspecting.
None of this can be know for certain, at least not by me. Bottom line, though, is that this sword likely isn't a treasure or even close.
Grey