It's been my experience that you get the nearest agent not where you turn in the mission, but where you are when the mission is accepted by the system. So, if for example, you turn in your mission and then go scooting off 5 jumps, you may get the agent closest to where you are at that point. It can take up to five minutes in my experience , from the time you turn it in to when the dbase accepts it. Hope that's not confusing you.
In that case you can certainly end up with an agent in another region. Or low sec as well probably.
If you really want a specific agent the best thing is to count your missions and make sure the 16th ends in the station where he is. And sit there until you get the mail from him.
Alternatively, as there are often agent only one system apart and you get a storyline from the one you don't want (SOE is bad for this), just delay the mission and you will get all the next storylines from the one you want until it expires.
Man... I'm tired and I hope I'm making sense