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One cause for what you've seen is if the search term is only contained within an image alt attribute. In this case only the alt text is shown in Google's snippet.
This is one reason that the description would be different.
Also, some sites don't have all that much readable prose for G to take. For instance, the page in question may be mostly graphical or have a lot of links and adds but no prose that has the search terms in it to display.
A CMS-driven site *should* not be any more or less prone to duplicate filtering by its nature alone, but code-heavy templates do seem to push you further towards tripping the dupe content filter, which is something to bear in mind.
>>show only 2-3 words as description in google
That can sometimes happen when people use a top graphic across the site to link back to the homepage with the identical keyword phrase in the alt attribute.
Two tables - one on top: graphic in left TD, same alt attribute text across the site, linked to the homepage. And text in the right TD - identical across the whole site.
Then a second table after with global navigation in the left TD and a little body text and a few graphics in the right TD (with the graphics in an embedded table).
It seemed Google stopped short right at that top table - problem with the description snippet. Which I've seen happen with a few sites.
I did a little bit of work for that site on just a few of the pages. What I did was unlink the top graphic, pull that top right TD identical text off and replaced with unique H1 descriptive of each of the pages.
Then I pulled some text out of the second table right side TD, wrote and added a bit more text, and put it in a blockquote in between the top table and the second table. Plus added a bit of text below that second table.
There were then normal descriptions next time around for those particular pages. Then they put identical text on top right of the pages again. :)