Unity Gain Phase Splitter- Why outputs have different voltages
To be able to achieve the maximum voltage swing at the two outputs we need to set the DC Q point at the "middle". The middle point is equal to \$\frac{V_{CC}}{2} = 7.5V\$. So we have two equal "halves" upper one and the lower one.

The lower emitter half we additional divided by 2 to get the full symmetric swing at the emitter output. Hence: 

\$V_E = \frac{7.5V}{2} = \frac{V_{CC}}{4} = 3.75V\$

We do exactly the same thing with the upper collector half and set the collector voltage at:

\$V_C = 7.5V + 3.75V = 11.25V\$  

I hope you now see why this is the case. 

If not I hope that this diagram explains it well.