Don't really get what's odd about this.
We know that Iran has at least a couple operational SU-25s. They were photoed a few months ago doing another training exercise.
It's not unusual to have aircraft loaded with live ammunition during a training exercise. It's "training". Every now and then, you drop/shoot live ammo. Plus, we don't know if they had live bombs or air-to-ground missiles. Just that they had bullets. Not unusual. Especially given the fact that tensions are hightened. Our AF regularly few armed aircraft during training missions in Europe during the Cold War, jsut in case something happened.
Also, it's INCREDIBLY hard to hit a target in the air with guns. The SU-25 likely doesn't have air-to-air combat taretting software or hardware loaded (even if available, given the statements about lack of parts, what are the chances they had them on these 2 planes?). This means that they were shooting with just hand-eye coordination. It's normal attack speed is around 400mph. Max speed of a predator is 135mph. That's a 265mph difference. Ever try to throw a bottle out the window of a car and hit a sign on the interstate? It's almost impossible without several tries. And that's at 70mph. Try it going 3.5x as fast! I'm not at all surprised they couldn't shoot it down. Even if it was flying level and straight, it'd be tough. If the operators of the Predator saw what was happening and took evasive action, you can forget about it. Plus, the predator is a VERY small target to hit. Could you throw a bottle out a car traveling 265mph and hit a rabbit that was running away from you?