It's nothing more than playing catch up.

If company X 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. Lowest is good if the mission is the same.

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 [matched that weight][1].
***

Here's why:
-----------

There's something called **Weight Factor,** which is SQRT(MTOW/50), MTOW in kg.

The Weight Factor determines the **[route charges][2].**

>This charge takes into account the distance flown and, less than proportionately, the aircraft weight.

*That's why it's vital for competitors to have similar MTOW's.* To market similar operating costs. MTOW is also used in airport fees.

The only interesting thing about **10,600 lb / 4,800 kg** is that the weight factor is kept a tad under 10.00 (two decimal places are used). Now, is there a higher charge for a plane with WF at or above 10? I scoured the internet, there doesn't seem to be WF brackets set that way, to my dismay.

Ryanair has part of their fleet tailor-made to an MTOW of **66,990 kg** (WF ~36.60) for similar reasons [apparently][3], or they're just being cheeky.

It's also why some airlines don't choose the Extended Range. The 777-200 and its ER version are the same, apart from extra certification and a **higher official MTOW.** The airlines can request conversion to ER later on if they open routes to farther destinations.
***
Side note, you'll find ICAO documents talking about standardizing said charges, so it's a global thing.


  [1]: https://en.wikipedia.org/wiki/List_of_airliners_by_maximum_takeoff_weight
  [2]: http://www.eurocontrol.int/articles/establishing-route-charges
  [3]: http://www.pprune.org/tech-log/494333-ryanair-mtow-66990-how.html