It is lost wort. You can do some math over your next couple batches and calculate loss on your system. I.e. if you get 6.5 in the kettle, boil for an hour and are left w/ 5.5 gallons, then lose .5 gallons in trub and transfer, you are left w/ 5 gallons drinkable beer. I found on my system I rarely got that loss (I got more) so I would get closer to 7 gallons in the pot if I wanted 5 gallons to drink.
"There's many a slip twixt cup and lip" -idiom