I don't know if this is true, but someone claimed that in the script which was just reviewed and apparently also leaked online that they "saved" his other arm and that it's a big plot point (a link to Robocop's humanity and all that stuff). There's also rumors that the new Robocop has many different "modes" and he can appear human-like when he's in public interacting with friendly citizens and doing PR, but when he gets into the "crime"-mode he grows that armor on and starts shooting.

In the original film they say "we were able to save the left arm", "I thought we agreed on total body-prosthesis? Now lose the arm!".

I have a hard time believing that's true (it's too hammy), but there could be a part where he's still having his real arm as a reference to that dialogue, like in the prototype/testing -phase (before they finalize "the product" and maybe test his shooting skills etc.) and they remove it later on in the movie. That would make sense.

But it's more likely Joel Kinnaman (who plays Robo) was just not wearing a glove when the photo was taken and/or it's not like that in the actual film.