I can assure you there absolutely is. And likely more than one reason. Like iWork is collected together, media is collected together, communication is collected together - but also just aesthetics. This is the layout that the designers preferred. Not just for the current icon set but historically as well, much of the layout can be dated back a long time, though some things have moved around a little.
But decisions like this aren't arbitrary even if the reasons come down to "I preferred it that way" it's not random.