Yeah, one of them has to be wrong.
The 6 vs 7 gallon measure is happening after lautering when I transfer the "7" gallons of wort in the bucket to the kettle. All of the liquid in both cases is at ~130F and both the bucket and the sight glass were dialed in at 120F (hot tap water) with the same 1 gallon jug.
The sight glass is pretty spot on for strike water volume and for pre-boil / post-boil volumes from 120F to boiling. Whatever it says is what has ended up in the fermenter. If it reads 4 gallons right before chilling the wort at 200F then it'll be 4 gallons in the fermenter at 68F.
I think the changes in density with temperature and SG are negligible with the ratio of water above the sight glass to the size of the sight glass. Or maybe it's in just the right spot (at the 3.5 gallon line) to balance that out. The temperature in the glass will be lower than the kettle. I've never had any of the problems people might associate with sight glasses - it's a fine piece of equipment and it looks cool.
I guess the solution is to reallly double check that bucket. Maybe by weight, got a weighing scale now.