Boil off, and losses due to wort transfer / trub. I prefer not to add water post-boil....to much trouble to get sterilized water. So, I adjust all my preboil volume losses in order to ensure that I have plenty going into the carboy/bucket. I lost 1.0 gallons per hour in my old 8 gal SS pot...its higher now, as I just switched to a keggle with its larger diameter (and much larger surface area). I haven't figured out my new boil-off rate for the keggle. So, I would think 6.25 into the kettle would be a little low, and you probably will lose more than 1.25 gallons before you get to the fermenter.
I prefer to ferment 6 gallons and bottle whatever doesn't fit into the keg. I have over 8 gallons going into the kettle and expect to lose ~2.5 gallons....most due to boil-off. In addition, I use whole leaf hops, and I loose some wort due to leave absorption.