It's normal to be able to lift more with a barbell than the combined weight of dumbbells. This is because you don't have to worry as much about stabilization. If you go to a machine, you'll most likely be able to lift even more, since your body is required to stabilize even less. Even with squats, there's a bit more of a balance challenge, so stabilizing muscles are used a bit more. That's one reason I prefer dumbbells for most exercises.
Also, when you use dumbbells, you are working each side separately, so the week side gets as much work as the stronger side. When you use a barbell, the strong side can compensate somewhat for the week side and help it a bit.
I also think, though this is just a personal theory, that sometimes using barbells causes one to not concentrate as much on the peak contraction. I'm thinking especially of bicep curls,where I feel a bit more contraction with the bells than with a bar. Some of this may be because with dumbbells, you can change the angle of movment a bit (for example, for chest presses, you can bring the weights together a bit at the top, which adds a bit of a squeeze. With a bar, you are locked into a specific linear movement.)
I suggest staying with the barbells weights you are at now for the next time you do PH, and really concentrating on the squeeze at the top of the movement. See if that helps. If you feel that you can increase weight and keep good form, then go for it! But it's best to increase the weight no more than 5-10% at a time. You might want to get some 1.25# and 2.5% plates for small increases (I have some nice rubber coatede ones from troy...the makers of the Troy lite system. I forgot the name of the company...has 1st in the title
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