555-timer circuit supposed to give 50% duty cycle square wave doesn't
If you look at the 555 internal diagram you can see that the output you use is not symmetrical: the top side is a darlington, so it has (at least) two Vbe drops. The low side is a common emitter, so it can be saturated. 



In short: the output low will be closer to ground than the output high will be to Vcc.

One way to compensate would be a low-valued resistor between the output and Vcc.