The lower-case form of delta can be written with that vertical leg either curving back to the left, or with a kind of sharp 's' curve to the right.
I picked up the habit of curving my lower-case d's to the left when I took a biblical Greek class, because it was easier for me to distinguish my own written Greek from a lower-case sigma (σ). I still write my English d's this way, for absolutely no valid reason.
You can see some examples of both delta shapes
here.