selenium webdriver - I capture the same web page as string for many times, but the screenshort strings are not always the same -

thanks take screenshot selenium webdriver

i wrote selenium test case , wanted compare special page's old image , new image. firstly, captured page image string , stored file.

string oldcapturedstring = ((takesscreenshot) augmenteddriver).getscreenshotas(outputtype.base64); 

i didn't change web programs. run selenium test case again , captured new image string.

string newcapturedstring = ((takesscreenshot) augmenteddriver).getscreenshotas(outputtype.base64); 

i found oldcapturestring wasn't sample newcapturedstring, 2 string same.

i want know why selenium captured strings not same @ different time, or made mistakes.

many thanks