I'm not at all surprised. There are some startups building dynamic pricing products that try to predict the optimal price for any given customer based on a number of characteristics (location, browser, device, payment method, 3rd party data, etc.). I don't know if Uber is using one or rolled their own but they're hardly alone in this.

Frankly, it's probably a bug in their system that exposed this. I would expect they'll start quoting based not on selected payment method but based on all your associated cards.