if I recall correctly, your manager gets XP based on the number of orders he OKs, not the number of items in the order, so the split up might be there as a measure to ensure that enough effort is being spent(and XP earned) when you queue up large orders.
Organizer, on my manager, is usually one of the first skills to hit legendary, even with the 30 item limit. (Miners in soil and moody dwarves aside.)
Basically, while each of those individual dwarves are getting XP when they do their tasks, the manager is getting XP every time they are assigned a new production order. So, while he's not earning as much or as quickly as a given dwarf doing the jobs, he's still approving a lot of jobs, and they do add up quickly. The number of jobs he approves will mushroom witht he number of industries you have up and running, and when some industries (like booze making) will 'take a break' as supplies reach an adequate amount, the manager still has orders in other industries to approve...
This will not be the case if you queue up repeating 'common' orders like "collect sand" at the workshop, as the manager isn't involved that way, but it's a trivial matter to re-order the production requests to make sure things stay on track, and the manager is almost always getting two or three orders to approve at once, especially once multiple industries are up and functioning.
Personally, I really HATE the 30 item limit, and would like to see large orders (make 200 barrels) as well as continual orders (keep making barrels until I say stop) and conditional orders (brew drinks, but only if drinks < 200). All of these would reduce the micromanagement of my fort to a degree, especially the conditional orders. I know you can duplicate the continual orders by loading a workshop with orders on repeat, but this is clunky by comparison, and requires me to track down my workshops instead of hitting three keys at any location (J-M-Q) to get the jobs started.