this is a youtubeclip showing the problem. And a description of the problem.
I think it’s definitely a weight problem. first, I notice that in the paint option, I don’t think that you have the auto Normalize button selected. Which means if you paint 1.00 with bone 1, and paint again 1.00 with bone 2, it doesn’t subtract it from the bone 1. so both of the bones will influence your object 50%. if auto normalize is on, when you paint 1.00 with bone 2, bone 1 which was at 1.00 will be changed to 0 and won’t influence the mesh anymore.
You can check if your vertex share some weight. when you have a prob like that, go to edit mode and then select one vertex with the problem and in the propertie window of the viewport, you can see under vertex group, how many groups affect the vertex you have selected.
Hope it makes sens.