i was reading tampa bay vs oakland play by play from NFL.com

i have a question about the following play by play

Third Quarter

3-5-OAK44 (:11) B.Johnson pass incomplete to J.White.
4-5-OAK44 (:06) J.Bidwell punts 33 yards to OAK 11, Center-D.Moore, fair catch by P.Buchanon.
PENALTY on OAK-J.Redmond, Defensive Offside, 4 yards, enforced at OAK 44 - No Play.

Fourth Quarter
Tampa Bay Buccaneers continued...
4-1-OAK40 (15:00) B.Johnson up the middle to OAK 39 for 1 yard (D.Clark).

1-10-OAK39 (14:34) B.Johnson pass to T.Brown to OAK 32 for 7 yards (N.Harris, T.Smith).

4th down and five, how come a defensive offside only penalize oakland for 4 yards not 5 yards? i thought it is 5 yards for defensive offside and give tampa bay a first down?

In reality, it was probably 4 & 5 1/2... so after walking off the penalty the offense was still short of the line to gain.

That's exactly it. Ball spots are rounded to the nearest yard, as are yardages-to-gain for the first. In rare cases, because of these two factors, and the fact that they do not EVER round to the zero (either at goal lines or first downs), you will see weirdness like this - and it will be statistically recorded like this. Add up all the penalties for Oakland, and this one will be "officially" counted as 4 yards.

See also - ball outside the 5, going in (but not outside the 5 1/2). Runner gains slightly over 5 yards, landing on the inch line. This runner gets "credit" for 4 yards, as the ball is still listed on the 1, officially.