I can fully understand tradsmen giving out estimates if the job is going to require some form of unearthing, where you ain't going to know what something is like until halfway through a job, like after removing a full bathroom of tiles, and having a load of repair work that needs to be done to the walls.
So, really, an estimate should only ever shoot up dramatically if something like this has happened, and it should be shown\ told to the customer.
Forgetting what half your job is going to entail though when giving an estimate is no excuse, like forgetting about trims, propper amount of adhesive\grouts that are going to be needed and so on.