How does Exchange 2013 Handle MX Priorities?

Occasionally I will run into an issue where a user is trying to email someone at a specific domain, and every other email they send bounces back (usually with an error of '550 relay not permitted'). Now in every case of this I have tracked it down to the recipients domain having a mx record that routes to a mail server that isn't configured for their domain (if I telnet to the IP on port 25 returned by the mx record and attempt to 'RCPT TO: user@somedomain.com' where somedomain.com is the name in question, it will give the same error, where the other MX record will work fine). 

Now what baffles me about this is that usually when this happens, the MX record that doesn't work has a much lower priority than the one that does. In the most recent case the working MX record had a priority of 10, and the non-working one had a priority of 30, but for some reason Exchange seems to round robin between the two MX records when sending email to that domain, instead of using the one with the highest priority.

Is this normal behavior for exchange, it seems to go against RFC 2821? We just have a standard send connector setup, using 'MX associated with recipient domain' and 'Use External DNS lookup settings on servers with Transport Roles', no smart hosts or anything. 

(running Exchange 2013 CU3 on server 2008 R2)