How very odd. It's possible (as a PP suggested) that Gwyneth got you confused with someone else. Or perhaps she misunderstood you (somehow?) and thought that you would skip the retirement party to attend Kathy's recital?
Or as cat-fu said, maybe Kathy is just trying to scam some bucks off people.
Whatever the case, you are clearly NOT in the wrong, and definitely not obliged to apologise for your non-attendance, or worse, pay them the $20 for a ticket.