One Tuesday that split my opinion on flat rate pay

Had a day last month in Phoenix where I fixed a cracked head on a 2008 Civic in 4 hours, then a simple brake line job took me 9 because every fitting rusted solid. The head paid 7.5 hours on the book, the brakes paid 2.5 flat, so I earned 10 hours for 13 of actual work. Some guys swear flat rate keeps you sharp, but that brake job made me lose $150 of my day to no fault of my own. Other mechanics in my shop said I should have upsold the customer on new lines, but that feels like padding the ticket to me. Are we just gambling every time we grab a work order, or is flat rate really the only fair system for skilled labor? Has anyone else had a day where the book time felt completely disconnected from reality? I keep going back and forth on whether I should push for hourly pay at the next shop I join.