I believe I registered my HL:Blue Shift key during beta, I think it bugged or something as I got every single retail HL1 product available at the time (and still have them).
I purchased HL2 on release like most people, was probably that.
So the first set of game(s) I actually had on Steam was the Half-Life Platinum Edition. It had HL, CS, TF as well as Opposing Force and Blue Shift. When I entered one of the keys it knew I had the rest. I think it may have thought you had the same set.