It's nothing more than playing catch up.
I'm company X and I managed to build an aircraft for a certain mission at the lowest ever MTOW. Company Y will try to get down to that MTOW to be able to compete.
Check the MTOW of the Airbus A320-100 and the Boeing 737-400, to save you time, they are 200 pounds apart. But the A320-100 didn't sell well, the A320-200 did. Guess what, the 737-800 matches that weight.
Now, there's something called Weight Factor. It's SQRT(MTOW/50), MTOW in kg.
The Weight Factor is used in calculating the charges the operator/owner pays, for taking off, landing, transiting, etc.
The only interesting thing about 10,600 lb / 4,800 kg is that the weight factor is kept a tad under 10.00 (two decimals are used in WF). Now is there a higher charge for a plane with WF above 10.00? I scoured the internet, there doesn't seem to be WF brackets set that way to my dismay.
For big planes, there seems to be, Ryanair has a fleet tailor-made to have a MTOW of 66,990 kg (WF ~36.60) for similar reasons apparently, or they're just being cheeky.