'67 Barracuda gas tank question: Why take only 8 gallons?

Background: Fuel gauge on the Cuda doesn't work, so my co-owner and I use the trip odometer to track how far we've gone since filling up. System has worked fine for years and we've been conservative -- never really needing to put more than 8 or 9 gallons in per fill.

We had some work done this spring and the mechanic reset the trip meter, and I thought that meant he had topped up the tank, but I was wrong. So yesterday I ran out of gas (for the first time in my 52 years). Fortunately I was close to home, so fetching a gas can was no big deal. I put four gallons in... wouldn't start right away... I could see clean gas in the fuel filters... and after some cranking, it fired up.

I drove straight to a gas station and was surprised when the pump clicked off after four gallons. Tank was clearly full.

Four gallons on the side of the road and four gallons at the pump.

Is the tank really that full of junk that it only takes eight gallons? How is that even possible and still have a running car?

Should we plan on pulling and restoring the tank?