Canonical ensemble describes system that is in thermal equilibrium with the bath of the constant temperature T. If bath has this temperature which is constant then system should have the same constant temperature meaning the energy of the system should also be constant. How is then possible that energy of the system fluctuates? 

It is also said (in my book) for canonical ensemble: "because system not isolated but rather in thermal contact with the bath of temperature T, the energy exchange is allowed and thus energy of the system can have arbitrary values". How is that possible if once system gets in thermal equilibrium it has constant temperature T?