Indexed pages : How does this number vary so much?

Ok, I have a number of sites that I perform SEO with, but I have always wondered what algorithms each search engine processes in order to come up with a number of indexed pages?

For instance, one site reports 78 pages in google and a whopping 119 in MSN. For starters, the site only has approx. 50 pages. Does this discrepancy have something to do with dynamic pages, or would it happen with plain html files as well? How can this number be so different from SE to SE.

Going from engine to engine will give you different results as they perceive your site's value in different ways, and may not index the same pages.

The dynamic pages would be likely to generate more than one URL for the same page, increasing your number of indexed pages. This should be fixed using the canonical tags introduced recently.

I'm reading up on using canonical tags, however do they apply to URL rewriting in the same way? I imagine they would.

Also, I assume this will not work for pages with different content, due to language. eg. you would not want to use a canonical tag for www.example.com/index.aspx?langid=eng and www.example.com/index.aspx?langid=fr. You would want each page picked up seperately.

Right, but those would be considered different pages to begin with.

The canonical tag is for one page (same exact content) that can be reached by more than one URL. It is different than URL rewrites.