What's happening is that your posts are merging. If you post on a thread where you have the last post and it hasn't been 24 hours, your newest post will merge into the last one.
As for bumping, generally:
If the thread is over 2 weeks old, don't bump it.
If you do bump a thread, make sure you have a good reason.
The market has it's own bumping rules found in the rules section.
In Offtopic you are allowed to bump anything so long as you are continuing the discussion or adding new information.