Simply...simulators suck. They bear little resemblance to reality.
I'd say, more correctly, sims work well within the normal operating environment of the aircraft they represent - that's the spec they're programmed to, and the programmed 'model' is unlikely to be accuarate much beyond that spec (why would the manaufacturer waste money making it so?).
Step outside that envelope, and you are almost certainly outside the mathematical 'model' within the sim's computer. The results will be unpredictable, and almost certainly not what the real aircraft would do (the further away from the 'model' you are, the more likely this is).
SSD