Hi there!
I think it is because these activities being generated by the QuickCampaign/Campaign are not directly related to the Lead in the same way. The regarding is the Campaign. We usually create plugins for these kinds of counts so we can include logic for those things not directly related.
Just to give an example, we created a this Activity Utility, which creates a direct relationship between the activity and the Lead, Account, Contact. This is basically a plug-in that does this, but if you did something like that you would be able to get an accurate count..