You may already have Pages v5.6.1 installed. Launch Applications : Pages (yellow icon) and from the Pages menu : About Pages. Is this still version 5.5.3?
If you originally installed the older Pages '09 from a DVD, it will be in Applications : iWork '09 folder. If you purchased it from the OS X App Store prior to Fall 2013, it will be provisionally in the Applications folder as Pages (with the blue ink bottle). Any installation of Pages v5 will silently move any non-Pages v5 application into the Applications : iWork '09 folder (or iWork '08 if using Pages '08).
Because you have Pages v5.5.3 installed, the first paragraph has taken place. Pages documents all look for the Application : Pages when you double-click them to open. With Pages v5.5.3 parked in your Applications folder, it will be the default application to launch — unless you left Pages '09 running, whereas it will be the dominant Pages application to intercept double-clicks — until it is quit.