Google descriptions

[Robocop]

Why does Google sometimes use the description from a web site as the text in their search results and other times used the Meta description?

[SkinnerW]

From what I've seen before for the new site if Google does not see relevant text right away they use Meta description. In many cases all they can find on the new pages is some kind of navigation links...

After spiders crawl the page they replace it with some actual text found on that page...

This is just my observation and I may be wrong...

[espmartin]

Matt Cutts explained that Google takes a "semantical" look at your content, and if they
determine a piece of text is relevant to a specific search term, they'll use that as the
description shown in their SERPs.

[smeagain]

Spot on espmartin, and that relevent snippet can be from either the the Description or from the Content text. The deciding factor is 'Relevence'.