I'm no 4e designer, but it seems pretty obvious what their logic was.
Almost every class has either 4 or 5 starting skills, and the distinction between getting 4 or 5 seems so trivial, I doubt that even a developer could answer that other than with something like, "it felt right at the time".
I think just 2 classes got less than that, which are the Barbarian and Fighter who only have 3. These 2 classes got less skills in 3E as well (the equivalent of 1 and 2 skills, respectively). 4E raised them a little bit so that they could have more options out of combat than 3E, but yet still considering them more about brawn than skill.
The ones that received more than the 4-5 that most every other class are classes like the Bard and Rogue, who again either inherited it from 3E or because it felt right because of flavor. Relative to 3E, they might have a fewer skills. That's likely so that other characters have more opportunity to take turns in the spotlight using a skill instead of delegating everything to the bard or rogue skill monkey, and also partially due to the fact that many skills were consolidated.
So, in a nutshell, skills appear to be designed in such a way that every player can potentially get a chance to be useful with a few skills, with only a relatively minor variation in number of skills classes have for the sake of posterity and flavor.
I'm no designer, and I can't speak for them, but this certainly fits in with the general design paradigm of 4E (which largely encourages teamwork, and sharing time in the spotlight).