It is possible that your down tube (and maybe even the seat tube and top tube) were SPX instead of SLX. SL wasn't all that stiff although both the down and top tube were .9/.6/.9. It isn't uncommon to have a slightly lighter top tube with other brands. A SLX frame alone (no fork) would weigh around 1900 grams + - (I'm going on memory) while SPX would be 2100 or 2200? That is one way to tell. A tubing sticker on the frame isn't always perfectly accurate.