Why can a UHF RFID antenna read a LF tag?
Generally RFID operates on near-field interactions. This is language that make something simple sound much fancier then it is. When you are in the near field of an antenna any coupling is due to capacitive and inductive coupling.

Most antennas for such a system are actually just large loops of wire and the tag often has a similar construction. The communication is exactly like coupling an AC wave through a transformer. So the different frequency still works through the transformer. The only issue you will have is if the frequency is too high to couple efficiently.