I had this occur to me recently as I had an Exchange 2010 environment and having installed my first Exchange 2013 server as a CAS & Mailbox. I tried logging into the Exchange Admin Center using the URL https://localhost/ecp and noticed that after I logged in it looked very much like the Exchange 2010 version of ECP.
After a quick search around TechNet I found that you need to add ?ExchClientVer=15 onto the end of the URL so its like this https://localhost/ecp?ExchClientVer=15 and it would then present the Exchange 2013 version of ECP which as you know is the Admin Center.
This happens if you’re in a co-existence scenario and your mailbox is still on an Exchange 2007 or 2010 server. See this TechNet article for all the info http://technet.microsoft.com/en-us/library/jj150562(v=exchg.150).aspx