Can someone explain this rule to me, the offsetting penalties situation in the 4th

[Laughing Coffin 1,342]

on the free play offsides, where Allen shrugged off a sack and hit Singletary in the flat, and then was nullified by a Knox illegal block in the back, why is it offsetting penalties there, and not at worst a 1st and 5 scenario? Especially with the Knox penalty being 20 yards downfield after Singletary already gained all the yards.

[seandelevan 17,487]

True...I thought for a moment the flag on the Bills would be assessed after the play and be a first down. They do this all the time in other situations but not this one. 

[Laughing Coffin 1,342]

ya i honestly don't understand, i thought the situation is at worst a 1st and 5 no matter what happens post play

[Buffalo13 21,983]

The Knox call was also a horrible call.  He hardly touched the guy.  But I have no answer about the offsetting penalties.  

[Bumbles247 38]

SECTION 5 FOULS BY BOTH TEAMS (DOUBLE FOULS)

ARTICLE 1. DOUBLE FOUL WITHOUT CHANGE OF POSSESSION. If there is a Double Foul (3-14-1-e) during a down in which there is not a change of possession, the penalties are offset, and the down is replayed at the previous spot. If it is a scrimmage down, the number of the next down and the line to gain is the same as for the down in which the fouls occurred.

[Nuke 27,156]

Yeah. I don’t get it. Why couldn’t the Bills decline the offsides penalty and take the ball at the spot of the illegal block penalty?