In Horford's last season in Atlanta, he shot 55.7% on 2-pointers. This season he shot only 52.7%. That is a drop of 3 percentage points, which is conveniently exactly how much his FG% dropped by (from 50.5% to 47.3%).
That data comparison is not very useful without looking into the shot profile for those 2PTers.
In his last year in ATL he took 30% of his shots within 3 ft of the basket.
In his first year in BOS he took only 21.3% of his shots within 3 ft of the basket.
He did drop in his efficiency on those 'at rim' shots slightly, from 74.5% down to 70.8%, but it was still within the noise of his career average of 72.6% on shots within 3 ft. That slight drop combined with the drop in 'at rim' frequency is what dropped his overall 2PT%.
Al shot better last year than he did in his last year in ATL and than his career averages in all the 'jump shot ranges' except for long 2s (16 ft to the arc). But he reduced his share of the latter type shot down to just 13.6% (from 23.1%).
Range '16 '17
0-3 .745 .708
3-10 .385 .472
10-16 .355 .392
16-arc .492 .450
3PT .344 .355
In the span of just a couple of seasons, Al has transitioned from shooting just 3.7% of his shots from outside the arc in 2014-15 all the way up to 30.2% of his shots being outside the arc this last year. That's a pretty dramatic change in his shot profile, as he is transforming himself into a thoroughly modern 'stretch big' who can handle the ball, create plays for others and shoot from deep. And he's doing it successfully.