why is my meta description changing?

well...probably it is a noob question.
anyway i have just noticed that when i search on google for my keywords...the meta description is changing.
so for my main keyword google is displaying one meta description (that i have written myself) but for all another keywords searched it is displaying the same result (my home page) but with different meta descriptions.....just like it was taking them scraping the most relevant part (the part that is containing the searched keyword) from my home page's text.

is this normal?

anybody?

Yes this is normal, google has been putting relevance on giving a description for what gives the user the most coherent results and with context.

For example: at one time, google never really put much weight into meta descriptions, however now they look at it, and in fact you might find your meta description being used when use do a keyword search on google, and is also a good measurement of how well and targeted your meta description is for the specific keyword.

Now depending on other keywords you have google will also give the abridged version if their keyword is predominantly in the 2nd paragraph you can expect that be the given result.

It's not that behind the scenes google is changing your meta descr, it's merely picking what it thinks is the most relevant information on your website for that given keyword.