a lot could depend on the angle you set the bench at. if its only a few degrees, then yeah your 'incline' numbers would be close to your flat bench.
also, think about your upper body posture... some people tend to stick out there chest and bring there shoulders back so even though they're on an incline bench, the ribcage is in more of a flat bench position... thus the strength levels are similar to flat benching -this is more of an ego thing..
it could also be due to learnt movement patterns. spending time on a type of bench pressing will benefit from the increased CNS input... have you been neglecting flat/decline and spending more time on incline?
lastly, if you're doing incline then flat then decline in a session, the fatigue build up will obviously hurt your performance as you move through the exercises.. if you train this way, try starting with flat or decline, and save incline for last....
i dont know if any of this applies to you but hopefully its useful