Mine does this quite regularly, generally if I start the car for a few seconds to move it on the drive, it returns to normal after a few miles on a normal journey.
Is this the first time you've checked it, if so take it for a proper drive and check it again, if it's anything like mine it will return to full.
It's like checking a dipstick immediately after turning your engine off, it would read low as the oil hasn't drained back to the sump.