Yes, the idles are lean because of the EPA. Open the screws more and a better idle and as well it makes up for the aircuts too. The aircuts are needed because idle is so lean that shutting off throttle at decel then backfires through pipes due to slow lean burn afterburning. When you open screws more the aircuts then are not even needed and can be blocked off to prevent their own special problems.
When you open mixture screws you are opening with throttle shut and high vacuum at the mixture outlet, the curb idle port. At full load and higher rpm the slides are open and that same port has much less vacuum at it and then the mixture screw effect is much less. As well, the primary main system and idle system are linked together, when the primary comes on line then the idle mixture output drops off and the curb idle port and idle air bleeds become extra bleeds that lean things out since again the curb idle port is then at a lower vacuum number, meaning the pressure has come UP in it. The primary outlet though will be then out in the active airstream and more pull on it at that time. The so-called 'seesaw' effect these do between the idle and primary systems, as one feeds less the other feeds more. Why if the idle fuel feed gets stopped up you are lean there at dead idle but as soon as you go toward the primary starting up then the mixture goes super rich, because the idle feed being plugged has the air bleed effect plugged too. Super lean then super rich dead back to back and why so many have fits figuring out just what the hell is wrong with the carbs.
Actual correct on the idle pilot jets is when you can open OR close the mixture screws and make the engine run worse either way, if it gets steadily higher rpm in opening them with no trying to go lower then you need a slightly bigger pilot, some go from the OEM #35 to #38 to get that. You consider the mixture screw setting the 'main jet' for pure dead idle only, then the pilot is a bit bigger to cover off idle until the primary starts up.
Yes, the lower load fuel systems feed at higher loads and rpm but depending on where the bleeds and discharge points are they can full flow at higher rpm or drop way off, every carb design is different in that. These the idle feed drops since the port is in the side of passage where at higher rpm there is boundary layer. The mains though discharge in the center of the airstream where the air is much faster. Idle flow drops as the engine gets faster and more loaded, in fact it drops hugely as soon as the butterfly edge opens up past the discharge point, at that point the pulling vacuum takes quite a dump.