Google and Yahoo sourced engines

[Decius]

Okay, something doesn't make sense to me (aside from the fact that the search engine world now seems to belong to Google or Yahoo!)
I tried one of those SERP checkers that queries different search engines to see what rank your site shows up in in different search engines. Well, here's a few questions:

1. Why are there different results on Alltheweb, AltaVista, MSN, and Yahoo! if they are all sourced from Yahoo! and/or Overture?

2. Why are there different results in AOL, Google, and Netscape if they are all sourced from Google?

3. What is the point of having all these search engines if they source information and technology from the same engines?

4. Are they simply tweaked in a simple way in order to depict the illusion of uniqueness?

5. Are the difference in results the result of delayed updates in the partner search engines from the main ones?

That's all for now...