gjp
As has been said "If in doubt, ask"
Personally, as an Area bod, I'd be the one to give you the 280 restriction.
Hopefully, the way it's given should help you understand why the restriction is applied though; for example if you look like being no. 1 from our direction with little sign of terminal delays you might be given "300 or more" which I would hope gives the impression of being 1st! Or, "fly at 290 knots" means "meat in the sandwich" Or, "280 or less" means nothing behind you to catch you up!
Alternatively, we could be looking at the stacks seeing the traffic building up and simply don't want you piling in hell for leather!