Outside of rule 2 having different definitions for each, is there a reason that distinction is made? (i.e, in what situation does the differing language matter?)
May not apply to Fed - I don't know - but, for NCAA, a backward pass that goes OB is OB at the spot where it crossed the sideline, period, and the game clock would not start until snap during the last 2 minutes of the 2nd or 4th periods. On the other hand, a fumble that goes OB in advance of the spot of the fumble is returned to the spot of the fumble, and the game clock is restarted on the RFP signal, regardless of time remaining in any period.
Terminology is important.