Quoting a job can still be tricky for mechanics, even though they work on the same model a number of times the job isn't always the same. It can be anything like a snapped head or exhaust stud or even spark plug thread inserts coming out.
I know a tank should be a pretty routine job but if there was a problem they should have told you why they took so long to do it, at ~$80+ an hour it's pretty easy to get a bill like that, still it would have made it a 10hr job. To change a pump on my old Falcon was half an hour.
I had to do some electrical work on oddly enough, a Triton, that was later sold to someone in QLD, I was going to look at something electrical that didn't work and ended up doing two full days work tidying wiring up on a car that was less than three years old. I had never seen such shoddy work and I had no instruction manuals for the million extra electrical items added to the vehicle. Luckily he had me look at it as it would have caught fire, there were about 5 unfused wires going in to the cab and the firewall grommet had been removed to make room for close to ten new wires.