Thanks for the advice everyone! I will try letting the engine idle for a while after a short drive and see if it makes a difference in the reading.
In response to Elly - I have always checked the transmission fluid by driving for at least 10 mins, pulling over, shifting through all the gears with the car still running, putting the car in park and then checking the fluid with the car running. The Subaru manual was not very spesific on how many minutes to allow the engine to warm up before checking the fluid, but did specify that the car should be in Park.