Variable pricing is a HUGE industry in the US (and elsewhere). It's true for fees like this, and it's even more true for variable rate mortgages. Whoever owns your mortgage can add arbitrary fees to your mortgage at any time, and whats more, is they not only don't have to tell you but the companies that manage the mortgage data consider the code and data proprietary.

I personally believe there is a huge, recurring fraud going on that eclipses what e.g. Wells Fargo has been doing to its customers.