When I post on an ANN thread or post a new thread it seems like it does not get put on the first page, rather the third. Is it based on a senority system where senior members posts are only allowed on the first two pages?
Thats not true mate. The post is based on the activity of your thread. If someone does commented on your topic post it goes to the top of the section with the exclusion of those that topic that are pinned. Those are special one, which is decided by the forum admin (maybe). Even newbies can have an active thread and it's not based on the ranking which post will be on the first or second page.