Unlike the NCAA, the NFL has rules on who has to line up where, as far as the 7 man line. There has to be an eligible receiver on the end of the line, and it makes it easier to note who is potentially eligible. An out of sequence number can be eligible if he "reports" to the Referee prior to the play, but the league's thinking is the number scheme helps.
In the NCAA, there just has to be 5 players numbered 50-79 on the line, and while they are never eligible receivers (there's no "reporting" and these guys by rule can't catch a legal forward pass), they can be on the end of the line.
Remember, however, that the guy the widest out may appear to be on the line, but he isn't. He's a step or two off the line, or was and shifts to a position on the line. Also, this doesn't answer all of your numbering quirks, but its just the way they think. The NFL also has numbering schemes for the defense which aren't explained by what I wrote here.
If a RB lines up on the line, is he considered one of the eligible receiver? How about a punter/kicker? How did they handle Randle El? Did he wear a QB number?
Thanks for the information, it just seems that there isn't a lot of differentiation between the skills player's....
"Unlike the NCAA, the NFL has rules on who has to line up where, as far as the 7 man line"
Are you sure about that? I have seen numerous calls in the NCAA for illegal formations(not enough men on the line=7)and it usually happens on punts,field goal attempts, and punts. I've coached Pop Warner and High School and the rule is the same, 7 men on the line with the Tackle covered. Unless the Tackle declares as an eligible reciever. I may be wrong though....