The original number mentioned in the specs of the phone would be for the entire "Internal" storage, which is used in part by the OS and any preinstalled apps. So you only have the ability to access/use the remaining storage.
The more things you put on the phone, obviously the more space used. Plus many of the apps also use up space through the items they put on the phone/store on the phone. Say saved messages, browser history and cache, etc. All that takes up more and more space.
You can add a SD card to store files on instead of the phone.