It could be an index error or a misuse of references, where product B is supposes to be product A + 10.

It could be that some local taxes or fees comes out to $10, and the client code does `price += taxes` on page load.

There could be code to round up to a $10 increment, and then subtract 2 cents so the prices are all $X9.98, but the rounding logic is incorrect. Since this is JS, they will be using floating point prices. That is a minefield in itself.

I'm not saying it's definitely a bug. But it trivially could be.