Strikes normally double the score of the next 2 balls thrown, spares double the score of only the next ball thrown. When the score is kept, though, I think the score of the next ball or balls are added to the score of the frame that you threw the strike in. So, if you got a strike on the first frame, then bowled an 8 and a 1 in the 2nd frame, the first frame would be worth 10+8+1, and the 2nd 8+1. Then a normal turkey is would be 10+10+10 in the first frame, 10+10+? in the 2nd, and 10+?+? in the 3rd, with the question marks being the score of the 2 balls you rolled after the 3rd strike.
If you work it out on paper, bowling a perfect game (300) would give you 270 through the first 9 frames, so the 10th is worth 30... they give you 3 rolls because if you get a strike on the first, then you should be able to get double points for the next 2 balls... same with the spare and doubling the next throw. Otherwise a spare and a strike would be worth the same in the 10th frame, I guess...
Hope I explained it right.