Welcome to brewing!
For extract batches take the gravity reading after diluting to 5 gallons and aerating so it is thoroughly mixed. If you take the reading before you dilute, that is the 'boil gravity' which is useful in calculating IBUs (hop bitterness, if you have gotten into the math on that yet).
Hydrometers are calibrated to 59F, that might have been a slight factor. The warmer the sample the lower the gravity reading.
Here is a calculator that helps adjust for that:
Hydrometer Temperature Adjustment Calculator | Brewer's Friend