There are any number of reasons this could be happening.
You play him at different positions. You could be changing the type of offense you play. Your opponents could be using different defenses against you. It could be the defender he's playing against is stronger/weaker. It could be the quality if his teammates and the defenders they're playing against are stronger/weaker. It could be his gameshape. It could be the number of minutes he's playing. And yes, players do go through streaks and slumps although it's quite possible that it's just the general randomness of the game engine that causes this.
But at the end of the day, don't expect your guards to put up very high shooting percentages. They're generally quite low across the boards on BB for a number of reasons.