Most heat pumps require 'special' charging techniques if adding freon due to the refrigerant management within the system. Now-a-days, if I suspect something weird is going on, I usually just recover then weigh in the new charge.
At the end of the day, you've got a 20*F split, so you can't complain about that.
If you want to pursue it further, and if after weighing in the correct charge doesn't change anything, I'd begin with the airflows. How is the air hitting the coil?