DC bias difference between square and sine wave?
The square wave starts out at a non-zero value. The sine wave starts out at zero. 

The time constant of 0.1uF and 5K is 500usec. If you look carefully at the blue line in your simulation you can see it trending downward. 

You need to either run the simulation for several milliseconds or figure out how to set the initial conditions so that the capacitor starts out charged to Vpp/2, not Vpp/2 -5V. I think there are a couple of ways of doing that in LTSpice.