How can one Bitcoin address be associated with mulitple IP addresses?
A bitcoin address is not based on or tied to a network address, they are completely different.

Think of the Bitcoin address as a bank account number and the IP as a telephone caller ID. When you call your bank from different phones they'll see different caller IDs but you'll give them the same account number each time. They can write down the caller ID if they're spying on you, but that doesn't have anything to do with the account number other than the fact that you yourself are using both.

Likewise an observer can watch you sending transactions to the network from your IP address, and if you use the same Bitcoin address more than once they can see your sending IP address each time (unelss you use Tor...). But that doesn't have anything to do with the Bitcoin address other than the fact that you yourself are using both.