BJT Voltage Divider Bias Circuit Theoretical/Ideal Model Help ..LTSPICE?
The main reason why is because the spice model is much more complex than the simple large signal model you are using, they are different. The spice model includes temperature effects and other small details that the simple model you are describing above does not. 

Here is a short description of what all goes into the spice model. In a spice model we want to model temperature effects, and other small effects. 

Source: Spice NPN model ECEE Colorado

Another thing to realize is both of these values would differ from the values of a transistor in the real world, because the beta of a real 3904 transistor even at the same temperature can be 100 to 300.