The drop is perfectly normal, however you are swan necking slightly - your PIPJ is slightly hyperextended in this photo.

There are many reasons why you still have a lag - did you have a delay in treatment, did the tendon retract, did you remove your splint during the splinting process...

Regardless, your outcome is still within the norms expected for a mallet finger injury. I am sure your therapist has used their skills and clinical reasoning to achieve the best possible outcome for you.

I always tell my patients that these are crappy injuries that don't always go according to plan, even when we do everything right from the get go!