If the current mirror was perfect then the 300uA current and the left-hand 294.1uA would be equal to each other. As things stand the mirror is pretty good with only 5.9uA difference between the two sides. A mirror can be improved with the addition of an extra two transistors to cancel out the effect of the extra current in the right hand side of the mirror due to the base current and also to make the collector voltages equal to each other to cancel out Early effect. If an extra transistor is added either side of the mirror then you will need to turn T1 into a Darlington (two transistors) to give enough head room for the extra left-hand mirror transistor.
At dc (quiescent conditions) there will always be a mismatch in the collector currents of the differential pair because T1 base needs some base current to hold the amplifier's output at the mid-rail voltage. However if the mirror is designed to be almost perfect with the left-hand mirror transistor collector current virtually equal to the right-hand diff-pair transistor collector current then you will find that the diff pair collector currents are very well balanced when an ac signal is being amplified.
This is the advantage of using an accurate current mirror, the diff pair collector currents will be well balanced with an ac signal. If the current mirror is not accurate or you use a resistor then, as you have found, the diff pair collector currents may be well balanced for dc but when ac is applied there will be significant 2nd harmonic distortion due to imbalance.
Another advantage of using a mirror over a resistor - It doubles the amplifier's slew rate.