If the output of the op amp is made of FETs, these FETs will have an ON resistance. When turned fully on, they look like a resistor up to the top supply or a resistor down to the bottom supply. If you apply a load the output of the op amp, you will pull current through these 'resistors' and this will cause a voltage drop across the FETs, thus reducing the output voltage. this is why most op amps that are specified as rail to rail are also specified when driving a certain (fairly light) load. The more you load them, the worse the voltage drop from the top rail is