SIP User Agents (UAs) are the end-user devices, used to create and manage a SIP session. Entities at VoIP endpoints capable of initiating and answering calls between peers, for example SIP phones and ATAs.
When you register to Asterisk, you do not say “him” what numbers you are going to handle. You just register to be able to communicate with him.
I think the problem is in the Asterisk dial plan (extensions.conf file), and we would need to have a look at it in order to figure out why these calls are being forwarded to your UA.
When you go for the SIP Account, the account is your register entry along with different contacts (different UA or same user agent having different alias). Might be the dial plan having the mapping of your sipua1 to 1234.