Lance may have better percentages last year, but he had the advantage of a better supporting cast and a smaller role in the team. Everyone knows that once you start looking for your own shot and taking more shots, your shooting % goes down, so it makes sense that Derozan shot a worse % while scoring 9 more pts/game. Derozan had a much bigger burden to create and run the offense, as shown in their usage% (28 vs. 19)
I would also say that at the moment, Derozan is a better ball handler than Lance. Lance is effective off the dribble at creating for himself and others, but is often out of control and turns the ball over at a much higher rate (Lance's turnover% is 18 vs. Derozan's 9.5). Plus Derozan can already get to the line at an elite level
Lance made a big jump last season from the year before, and if he continues to develop his game he could easily be better than Derozan this year. But he is not better at the moment
No way in hell Derozan has better handles than Lance. Lance can play PG position is he had to. Lance has some of the best handles of any SG in the league.
If Lance took near 20 shots a game and was the main / only option he would average 20+ points as well.