Yeah, I'm not sure exactly what's going on. One thing to remember, however, is that the game is certainly rounding numbers off a lot and that's probably happening in those screens too. Of course, this still doesn't account for the tax revenue being only 297k, since even 84.5% of 50% of 720.5 results in 304k, so I'm not at all sure what's going on either.
That being said, you should remember that you're Dhayut. Your growth is meh, your tech rate is limited, your economy is nothing special, but you've got some insanely good warp drives, which should give you quite the scouting advantage. And more mobile fleets count for a lot. Attack one place and when reinforcements turn up then simply go somewhere else. Humans have the typical crap drives for a very long time so your mobility advantage is going to be massive for a long time.
And don't be too intimidated by their fleet sizes. The AI tends to have many relatively weak ships and fleets with relatively crap weapon assortments and all you can tell is that they've got a total firepower that outranks what you've got, but firepower in itself isn't too important. It's simply the alpha strike value without any consideration of DPS or how easy it is to bring to bear. If you have a single fleet that can stand against any of their fleets then your mobility superiority (and presumably your much better ship design) is going to balance things out quite a lot.
By the way, one little sort of cheap thing that I've done to great effect against technically superior AI navies is constant movement of my fleet inside a star system. Move to the star in the enemy system. A bunch of ships will appear around you. Give a movement order towards some planetary object in the system. Your ships will move in the direction of that planetary object, attacking one group of the encircling enemies while leaving most of the rest out of range. Your ships will jump before taking too much damage but the small group of enemies that got crowded by all your fleet will have suffered a lot of damage. Wait for the enemies to start appearing around your fleet and then repeat the maneuver.
What you're accomplishing this way, aside from having a sensation of tactical combat, is local numerical superiority and while this game isn't Rome: Total War, having numbers on your side in smaller local engagements while denying an all out clash between forces goes a long way towards neutralizing the advantage of a stronger enemy. And yes, I'm essentially saying that you should go on the attack instead of being defensive. Humans grow faster than you anyway and if you're defending then you're not expanding like a nasty bug infection.