Rules question

[gobuxx69]

This has happened twice in Cleveland Browns games this season. In the Baltimore game it was third and three from the 11 yard line, Baltimore had the ball, the runner gained 5 yard to the 6, after the play the runner kicked a Browns player, a flag was thrown and the referee, said "after the play , unsportmanlike conduct agains Baltimore,d penalty and then moved the ball from the 6 yard line to the 21 and said first and ten.

In the Washington game it was third and 8 yards, a pass was completed with enough yardage for a first down. After the catch Pierre Garson was penalized for "taunting" The refereee said, "after the play, a 15 yard penalty against the offense for taunting and marched off 15 yards, now first and ten.

My question in both of these instances is, if a first down had been made, and now a penalty is thrown, why would it not be first and 25 yards instead of marking off a 15 yard penalty and then saying first and 10? Needless to say they lose 15 yards either way, but it is MUCH tougher getting a first down after a first and 25 yards, instead of just moving back 15 yards and then claiming first and ten?