Well... I'm the kind of guy who had this same doubt for months and months. Have a look at my NL4 graph here:
Operation Ace of Clubs - Poker PRO by Dec. 2011 Business Venture
I did not understand what non-SD winnings were all about until I read two books: "Let there be Range" and "The no-limit Hold'em Workbook". They teach you how to understand your own hand ranges and how to evaluate your opponent's ranges, basically.
By doing that effectively you are going to be able to c-bet bluff when the board texture is appropriate, for example. When you do that and succeed, you have a non-SD winning. You will learn to raise the river with second pair too, for example. Maybe in this situation you had a correct evaluation of your opponent's range and decided that he could not withstand this particular raise. If you succeed, you have more non-SD winnings.
That's quite simple to understand. Have a look on those books and your game will change A LOT.
I also had this same problem you describe before studying hand ranges.
