When the "Y's" are next to one another, you have a choice to make three or four smaller locs out of the four branches at the base. You can combine the two shorter branches of the "Y" into one loc and then keep the outer branches as two separate locs giving you three locs out of four branches.
Why split the locs? Again, smaller locs can seem to offer more styling options.
Pictured is a "Y" that is about 3 inches long and ready to be split.