I think it has something to do with whatever they were born on - Bastet was born on newspaper and you guessed it - no newspaper in the house for her to pee on. If she had the chance she would also pee in the bathroom but knows not to do that anymore for fear of the evil spray bottle monster!

Bastet's mum decided that the bathroom tiles on a pile of newspaper (that was going to be used for wrapping up the broken tiles when they came off the walls) was an excellent birthing place. Her previous litter was born in a potplant holder (go figure).

This is just a theory mind you but so far it seems to ring true for pee habits of cats (that I know of anyway).