The first floor of a large building is too big an area to designate as the location of a human being. It would be evasive and obscure to indicate your location so largely. Plus, inherent in the question, "Where were you?" is the attached question: "What were you doing?" And especially if you were being accused of committing a horrific crime, you would want to be as explicit as possible about where you were and what you were doing.
So, if Oswald were asked where he was at the time of the murder, and he just said he was on the first floor, without saying exactly where he was on the first floor and what he was doing, it would sound an alarm that he was withholding something and being duplicitous.
So, why would Oswald, who was innocent, want to make an impression like that? He wouldn't. So surely, when they asked him where he was during the murder, he told them precisely: the doorway. And it was on the first floor. And the doing, of course, was watching the motorcade.
But, speaking of withholding something and being duplicitous, that is what Hosty and Bookhout were doing when they said that Oswald said he was on the first floor during the motorcade. It was too big an area, and it included no reference to what he was doing. Oswald surely would not have left it like that, so why did Hosty and Bookhout?
It's because the rest of Oswald's statement was that he was out with Bill Shelley in front, and they didn't want to include that. So, they left it out. But, what they said wasn't technically a lie.
However, the deficiency of their statement is just as glaring as if Oswald had said it- but the duplicity applies to them, not to him.
Why wasn't it noted at the time? Because there was no will to do anything but see everything in the light of Oswald's guilt.
I'll say it again: Because there was no will to do anything but see everything in the light of Oswald's guilt.