Maybe they felt switching to 1 would feel like too much of an effective increase of the character limit?
Unicode, some emojis are comprised of two characters (two emojis) put together, thus are two characters. Often one is the main character, and the second is the variation code.
Thanks, I happen to know about the combinations (they're also called out in the article). But the multichar ones are combined with more than two. So if you can consider four to be two, why not simply consider four to be one.
The only logic I can think of is that the equivalent text based emoticons - :), :(, :D, D:, etc - are usually 2 chars.
That's pretty good haha. Though there are exceptions it's generally true.