- Are all Ubuntu download servers an exact copy of the main server?
- Can a software be on a server but not be on another server?
- Can the version of a software be different on different servers?
- Where is the main server?
No. Servers need to get updated from the main server so some repositories will lag a little bit. Here is a list of mirrors with a column that states how far behind a server is. As you can see some are a week behind but you might also expect that this is due to localisation (translations in progress).
See 1. In general this is only the case when a server is updated and that mirror is not updated yet. I would assume this to be the case when a new release is out. The mirrors will need a bit of time to update.
See 1 and 2.