javascript - Why is the viewstate sent to the server on form POST different than the Viewstate in the form html? -

i attempting scrape asp form. under impression that, in order form post accepted, had send fields of form including viewstate, eventvalidation etc. when this, though, go error page.

looking values submitted browser, viewstate sent server different viewstate in form's html. after searching everywhere think of, still can't figure out why is.

so, question is: why viewstate sent server different, , how correct viewstate?

(this more of conceptual question why haven't posted code, i'm working phantomjs if makes difference)