It goes against her slave training, which was done through a very cruel system where slaves are treated less than people (than maybe nobles' pets), so they learn not to expect anything good. So when faced with a master that does not condone such inhumane treatment and instead wants to her to be "normal," it's an extremely difficult thing to break out of the training regardless of her new master's orders.
To give a loose example, it's similar to a long-term prostitute that has been brought off the streets and was started on rehabilitation. However, the prostitute has a mindset geared towards money/survival that is so deeply ingrained that the prostitute is struggling in rehabilitation and would prefer to go back out to the streets to make "easy" money despite the dangers.
Hopefully that makes sense. It might be a bit too convoluted, so maybe someone else can give a better example.
If she really had a slave mentality then she wouldn't have questioned her master's orders so many times and made him repeat himself so many times, right? Or am I weird for thinking that? ┑( ̄Д  ̄)┍