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With extract brewing, gravities are very predictable unless you spill or leave lots of wort behind in the kettle.
Your post boil volume is the big variable, but all the sugars are still there, more or less concentrated.
You don't need a violently rolling boil, a good simmer (rippling on the surface) is plenty. Boiling off half a gallon to a gallon an hour is very average, and with smaller batches or boil volumes that's a significant amount. Now topping off in your fermenter to your intended batch size should fix that. After topping up, stir extremely well and thoroughly to homogenize the wort before taking a gravity reading.
Especially with partial boils, don't boil all the extract, only a part (half or less) and add the remainder at flameout.