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. But after some time when you login your account again and check your activity again you may see the correct account of your activity. If you see positive karma in your post it means maybe some member liked your post and your post was informative so that other member learned something new by reading your post and because of learning something new he gave you positive karma. Hope this short discussion has answered all your questions.