I'm very confused with pro sites. I'm doing several tests

I'm very confused with pro sites.

I'm doing several tests to try to understand it.

1) I subscribed today to a Monthly Plan. The last payment day is August, 12, 2014 (because I have 1 trial day). The next payment day is September, 13, 2014.

2) After, I did an upgrade and subscribed to other plan (an ANUAL plan). The first payment day is May, 13, 2015. Why?

Since the last payment date is August, 12, 2014, from august to may is not 12 months.

Please, help me, I'm not understanding pro sites.