Hello,
This could happen if on the web server there is a different flavor of the updates setup than the one added in your updates configuration when you built your updates configuration file.
Every time you rebuild the updates setup you should also rebuild the updates configuration project and then re-upload on your server both: the updates setup and the updates configuration file.
Also, you can check my colleague Dan's reply on "download update size does not match" thread too.
All the best,
Daniel