Anything can and usually does take more time than either party might initially think it will.

As an example.
I was replacing all the under-body brake lines on a 1970 Challenger. And I had to remove the rear axle vent valve and block.
Problem arose when the valve was seized in the block. So out came the mapp gas torch to heat it up just a tad, an adjustable wrench to hold said valve. etc etc

What should have taken 5 minutes to replace took nearly an hour. Sorry but no one can plan for that.

And an estimate is just that,,,,an estimate.

I try to estimate everything I do. And when it comes to old car work,,,,,all my customers know before the 1st screw gets turned. It's an estimate and not a guarantee written in stone.