I don't think it is an underinking error. My reason is that the front and the back of the note are printed at two different times. So it would be much more unlikely that this would happen to both sides. However I do know this: the ink used on the seals and serial numbers is different that that used for the rest of the note. Perhaps that type of ink is more resistant to wear? Then again the note looks pretty evenly worn. I'm interested to hear what other people think.