Well as a guess I would say that maybe your deltoids are more developed than your chest ? Dumbells use alot of stabilizer muscles, while Bench Press tends to rely less on them, and isolates the chest more. It could be the case that maybe you just don't feel comfortable with a straight bar.
On a side note, I bench 505, and struggle with the 150 lb dumbells for flat bench lol.
I would think your bench could come up to what you press with dumbells with a little more practice.
Mighty