aircon power draw slowly increases

[George]

I have an old box aircon. I put a power meter on it. Turned the knob to
maximum cold, so compressor should stay on full-time (I am in southern
hemisphere, summer here).
Power draw was initially 590 watts, and it gradually increased to 690
watts after 5 minutes. It is not a modern inverter system, but an old
one where the compressor is just on or off. So what is going on? Is the
refridgerant warming up, so compressor has to work harder?