Let’s say your boil starts at 6 gallons. You have 120 points of fermentables (e.g. 5 pounds of 30 ppg at 80% efficiency). You boil down to 5 gallons. In this example, your system may lose 1 gallon to kettle dead space, hop absorption, losses to the way you transfer wort, wort left in the chiller, and so on. In this case, you up with 4 gallons in the fermenter.