Why do newer 737s use two different styles of split winglets?
Since a winglet is basically a sail generating thrust (lift with a modest forward-tilted vector) from the circulating flow around the tip, extracting beneficial energy from the circulation (the thrust part) and producing an "outwash" that opposes and weakens the vortice flow in the process, it's just adding another sail beside the first one to extract more of the available energy from the flow.  From the NASA Dryden page:


  Winglets, which are airfoils operating just like a sailboat tacking upwind, produce a forward thrust inside the circulation field of the vortices and reduce their strength. Weaker vortices mean less drag at the wingtips and lift is restored. Improved wing efficiency translates to more payload, reduced fuel consumption, and a longer cruising range that can allow an air carrier to expand routes and destinations.
  
  To produce as much forward thrust as possible, the winglet's airfoil is designed with the same attention as the airfoil of the wings themselves. Performance improvements generated by winglets, however, depend on factors such as the basic design of the aircraft, engine efficiency, and even the weather in which an aircraft is operating.


In the case of the MAX, you can see that the lower winglet is at less than a 45 degree angle, so it's producing mostly vertical lift with a slight forward component because as you can also see its incidence is somewhat nose down relative to the main wing to optimize its AOA in the tip circulation.  It's kind of a half winglet, half tip extension.

Different engineering groups will do studies of different configurations and will say, "hey, if we do this, it'll be some little bit more efficient than if we do that, based on our particular analysis".

If you put different groups of engineers together to attack the problem, they are all going to come out with variations on what they think is the ideal configuration.  And so you see seemingly endless permutations that are, really, mostly nibbling at the margins of the major benefit that was achieved when you put one there in the first place.

Plus there is probably a bit of "styling" going on as well.  Just to be different.