It works like this. If you have a history against an opponent then their complete stats will show any time you play a SNG against them. For instance, if you have played against them over 300 hands in however many SNGs, the stats will be based on all those hands.
However, your own HUD for the particular SNG will be based only on the hands you have played in that particular SNG. I guess this is to give you an idea of how you have played this particular game, which in turn should give you an idea of how your opponents will view you. You can then base your game on how you think your table image is.
Of course, if someone else at the table is running the software and has played against you before then their HUD for you will be more comprehensive than your numbers for that particular game.
Hope this makes sense.
Cheers, MB.