Calibrate your buckets, carboys, fermentors whatever. If you are relying on some factory marking on a large volume container to tell you the truth, there is an excellent chance you are way off.
If you do not have an accurate gallon measure, buy a gallon of milk, mark the level of the milk with a sharpie, and this will be your reference gallon. Fill your buckets, carboys, whatever, with water a gallon at a time, and mark them. Do the same with your brewpot, fill a gallon at a time and make a dipstick out of a piece of wooden dowel.
You are bottling 3.8 gallons of beer. Pretty hard to "lose" a gallon of wort/beer. But a 5 gallon original wort could easily "shrink" that much due to trub, racking loss, bottling loss, gravity readings/taste testings, and incorrect volume measurements using large containers.