A post is equal to an activity, but your activity will not increase immediately after posting, when you post, you will see your previous activity at that time.
Incorrect. Activity depends on posts but is not equal. For instance the teleported ones can get activity without having enough posts and some posts are not counted as activity. If your activity didn't grow at the moment when you left a post, it won't grow from this post in the future, only from the other posts which will be fitting for activity rise.
Teleported accounts are usually given activity based on rank but the accounts created by this forum as far as I know account rules that when a member posts a post, that post is added to an activity. I'm sorry if there's any mistake in saying this because I'm trying to say as much as I know here. Your post count may not match the activity until or after teleporting, but I know that one post of account teleporting usually results in one activity. I hope that as far as I know, there should not be any mistake in this information, but if there is any mistake, I hope that the mistake will be seen with forgiveness.