Manager’s don’t estimate workloads.
They challenge the estimate, playing devil’s advocate, poking holes where they see them, but they do so with the goal of helping the developer get better at the task in being more precise.
They don’t do it to knock them down or crunch their numbers furthers.
There is no mythical person month that makes work go faster. If you want it done right, you put in the time and you get it done.
They insulate failure for their team that are providing the estimate.
If your Manager is estimating your work for you, chances are the number is wrong.