A self-bow may loose some weight if kept strung, but not a glass laminated bow. Your brace height probably dropped.
A change in temperature and relative humidity can affect bow weight a little, as well as the scale's functioning. You may have just read the scale a little differently. A 27 1/2-inch draw on a 56-inch recurve or hybrid is not over stressing the limbs. May start to stress a straight limb bow a little, but again, wouldn't affect the poundage unless it's a self bow.
Regardless, not something to worry about IMO.