This can be a confusing issue for structural engineers. The 125 is what should normally be used in "office" type environments where a storage closet, or room, is designated on the initial architectural design documents.
The problem is, of course, that the designated room may move someday and a file cabinet area may appear in what would normally have been a cubicle city area.
We usually try to pin down the storage areas initially, making sure that the owner isn't intending on using dense file storage cabinets (200 to 300 psf).
Also, the floor system (say a deck and concrete floor) can and is usually designed to accept higher loads - look in some of the SDI tables and you will generally see numbers in the 100 to 400 psf range. If your deck can take locally high pressures, say from a group of heavy file cabinets, the overall joist or beam system can usually accept these smaller, high-load areas without overcoming the beam/joist capacities.
The problems come when a few years later the occupant puts in a library of high-stack books, an MRI unit, a dense file cabinet, or simply loads up a room with boxes of paper. That's when the strengthening is required.
250 psf storage, I believe, is intended for those higher stacked warehouse floors, not for 12' floor-to-floor situations in office buildings.
