Theoretically, you're getting the form that compliments your playstyle. If you're ganking Top or Mid (where most melee characters are), you're usually going up against bruisers and assassins who need you to have that extra CC and tankiness and %HP damage to deal with them. Thus, if you're aggressively camping those types of characters, that's why Rhaast is the first one to unlock for you, and will probably benefit you more in the game you're playing.
The flipside is also true. If you're ganking bot a lot, as well as the mid lane mages, that means you're valuing taking out the carries more than the front liners, which is why Shadow Kayn will fit your playstyle better. If their other lanes are also more composed of ranged units, this is also good for Shadow because that means an assassin playstyle will do better than a tankier one against those foes.
There are a mild few exceptions of course (Graves, Jayce, Urgot), but for the most part, if you're getting more of one color due to how you're ganking, I believe that the corresponding form will compliment your style of play better for that game.