Trying to dis-assemble something that is rusted/corroded without destroying it- that is sometimes not enough time
Labor units - if this is one panel in a large project some of the preparation time, materials handling, waste disposal, etc., may be somewhat combined with other items/tasks making what labor specifically applies to this one panel less then if the entire project were to install just this one panel. The time to mount the panel, land the supply conductors and wire up the branch breakers stays about the same either way.
If you were doing 20 of the same panel with same branch circuits on 20 different floors or in 20 apartments or something like that - the repetition of tasks factor probably makes the first one go slower then the last one.