Let me try to clarify. A wind back / lockout / crank is very accurate, and more accurate that a drop weight or an electronic machine. If a lockout is not dead on it should be adjusted so it is. Once adjusted it should pull to the exacta tension every time. Set it to 60 it pulls at 60 time and time again. Sometimes a lockout will drift off and needs to be calibrated. My Gamma was usually dead on. Over the course of a string job if you have any noticeable drift something is bad wrong.
When I use my Wise tension head, it pull tension and overshoot tension by usually about 0.1 lbs then it stop pulling. As the string relaxes the tension drops when it is -0.2 lbs below the target tension the tension head starts again and the process is repeated over and over. So when I string at 60 lbs and clamp the string the tension could be anywhere from 60.1 to 59.8 lbs and the longer it takes to clamp the string the more the string is clamped under that constant tension. Therefore the string is stretched more or less depending on how long it was stretched. Even though the tension remains constant the stretch is greater resulting is variations.
BUT i prefer to use an electronic tension head. The reason being is because of the slower continuous pulling will result in a string that hold tension better and allow you to string faster.
I would like to apologize to anyone I have not offended. Please be patient I will get to you shortly.