Why can i do more on seated dumbell military press compared to standing barbell military press?

It all has to do with gravity and balance. When you're standing vs. sitting, you take energy away from the exercise and send it to your legs. That and if you're not sitting completely straight up at 90�, it's not the same angle and energy is also being used in a different way. Another reason is that when you use a barbell on a track, it's focusing energy to one specific area of muscle. When you use dumbbells, you have to create extra energy to keep the free weight in place and balance the weight you're holding, instead of the track doing it for you.