iPhone automatically adding contacts to blocked list

My iphone 5s is automatically adding numbers to my blocked list. I have to go into the settings menu every day to clear out the list. Sometimes there are numbers in there, sometimes there are not. It doesn't seem to have any preference as to whether I speak to the contact regularly,or haven't used the contact in a year, nor does it care if I pick up the call or ignore it. I've checked the contact and it is not blocked from the contact screen. It will also put random numbers that call me (that are not stored in my contacts) on the list.

My phone has done it since I bought it in April, and my boyfriend just got one last week and his does it too. Has anyone had this issue? Why is my phone deciding who I can and can't talk to?