You don't say what model your tractor is.
The sight glass reading is very dependent on the tractor being level. Also the reading is dependent on weather the fluid is cold or warm. In the summer I park my tractor in my pavilion for easy access when mowing a lot. There is hardly a noticeable grade - the tractor will not move when off and in neutral just the same as level ground. The site glass always shows no fluid when parked there. But if I bring it into the barn the level shows just right half way up the glass.
And like others have said, if you have attached any hydraulic implements for the first time, they will take some fluid away from the system. The proper way to check the fluid is when the tractor is cold, any loaders etc flat on the ground, and the 3 point hitch lowered.