I think your parent is trying to say that you won't hire the plumber that only tells you he will be efficient doing his job. But you will hire the plumber that tells you "if I don't find surprises behind the wall, this is a 2 hour job". And then he opens up the wall and finds old corroded iron piping he has to trace down to where it attaches to the main line and replace it all or else you won't get your new bathroom done at all.

And that job takes more than 2 hours. You could also have said "well if that's the case can you poke some exploratory holes in the drywall please so I can get a better estimate" (aka a "spike").