Cribbage scoring rules explanation
So I'v been googling like crazy last hour or so, and here is what I found. A person on BGG recommended pagat.com for comprehensive coverage of old public domain card games. Indeed there is an article there covering cribbage. 

I'll cite the relevant part:


  The Show
  
  Players now retrieve the cards that they put down during the play and score for combinations of cards held in hand. First the non-dealer's hand is exposed, and scored. The start card also counts as part of the hand when scoring combinations. All valid scores from the following list are counted.
  
  15:
  Any combination of cards adding up to 15 pips scores 2 points. For example king, jack, five, five would score 10 points altogether: 8 points for four fifteens, since the king and the jack can each be paired with either of the fives, plus 2 more points for the pair of fives. You would say "Fifteen two, fifteen four, fifteen six, fifteen eight and a pair makes ten".
  
  Pair:
  A pair of cards of the same rank score 2 points. Three cards of the same rank contain 3 different pairs and thus score a total of 6 points for pair royal. Four of a kind contain 6 pairs and so score 12 points.
  
  Run:
  Three cards of consecutive rank (irrespective of suit), such as ace-2-3, score 3 points for a run. A hand such as 6-7-7-8 contains two runs of 3 (as well as two fifteens and a pair) and so would score 12 altogether. A run of four cards, such as 9-10-J-Q scores 4 points (this is slightly illogical - you might expect it to score 6 because it contains two runs of 3, but it doesn't. The runs of 3 within it don't count - you just get 4), and a run of five cards scores 5.
  
  Flush:
  If all four cards of the hand are the same suit, 4 points are scored for flush. If the start card is the same suit as well, the flush is worth 5 points. There is no score for having 3 hand cards and the start all the same suit. Note also that there is no score for flush during the play - it only counts in the show.
  
  One For His Nob:
  If the hand contains the jack of the same suit as the start card, you peg One for his nob (sometimes known, especially in North America, as "one for his nobs" or "one for his nibs")..


While not entirely scientific, this is the most unambiguous description that I've come across so far, and it also acknowledges a feeling that I was getting that this particular rule is slightly illogical. Italic above is mine.