Why Google, Yahoo, MSN differ in one way or other

I've seen many-a-post from people asking variations on these questions:
* Why does Google show fewer backlinks than Yahoo?
* Why does Yahoo have fewer pages indexed than MSN?
* Why are my SERPs changing more in MSN than Google?

I am no expert on all three search engines (SE) but I do want to provide an opportunity for everyone, newbie and veteran, to learn about why these differences exist... to add their input to educate us all.

Let me begin with this:
Why are there differences? Don't forget the most obvious thing -- Google, MSN, and Yahoo are all difference search engines run by different companies, setup with different algorithms and processes, running of different servers, etc. They are different, so don't expect them to mirror each other exactly. If that happens, then competition dies because we have clones, not competitors.

BL's Show Different in One SE over Another
Google intentionally hides some of the information it knows. MSN and Yahoo are more willing to tell you what BL's it finds as soon as it finds it. Google knows about most to all of the ones Y and MSN show, but chooses not to publicly display them all. Why? They want to prevent spam.

Yahoo & MSN may not be exactly the same, either. That's because it depends on when the SE crawl certain sites, how often, and what the sites have on them at the time of the crawl. MSN may show 100 more than Yahoo one day, then the next week Yahoo has 40 more than MSN. It will change as links are added and removed, as sites are indexed/de-indexed.

Differences in # of Pages Indexed
I'm not exactly sure why, but I do know Yahoo does tend to show the fewest pages indexed. I invite someone to add to this part for me... because I have no idea why, except that Yahoo isn't Google.

Other Commonly Asked Questions about the Differences
Feel free to add questions you see asked a lot and provide the answers to them. I hope this thread becomes one that is highly referenced in the future.

I agree with most of this. Basically they are different becuase they are different. Certainly Google hides much of what it knows, to deter reverse enginering, slow SEO research, and of course, minimize SPAM.

The differences are there to becuase the searchers are different. Customers I get from AOL searches are different from Customers I Get from MSN searches, in a general profile way.

It's been said that Yahoo is more popular in B2B searching & Google is more popular for B2C users. This is all opinion though, I haven't seen anything concrete to support it. I do know my conversion rates are Way higher for G than Yahoo...MSN is still a work in progress.

I found Y! shows way more pages indexed than the others. Ever since they claimed the largest index.

Take my site. 77 pages that Y! claims is more than 100. What are these mystery pages? I have no idea. The have nonsense names and never actually existed.

Another difference is G supports more file types than MSN and Y! And gives more weight to them. For example .pdf, .doc and .txt Y! seems to be going after the multi-media market.

MSN refferals have the highest number of conversions once they reach the site.

G has a sandbox.

BLs count for more in G than the others. Onpage factors seem to have the biggest impact on MSN from what I've heard.

KW density doesn't work in G, but does on the others (one more than the other, I can't remember which).

G ries to get you off their site ASAP. MSN & Y! try to keep you around.

MSN & Y! are ahead of G supporting RSS.

...

I'm sure the core of each search engines algo is pretty similar, (there's been enough engineer poaching) the differences seem to come with the filters they apply afterwards. Google leads the way with filtering serps with their sandbox and heavy filtering for site-wide backlinks etc etc.

With Yahoo and MSN jumping on the adsense contextual advertising bandwagon, I wonder if the filters in the future will just be to stop spam or will commercial filters start to appear.

Those major search engines Google, Yahoo, and MSN(Bing) are having different features and style of crawling every site. this may be the big difference among the Three.