Win7 SP1 2X Asus Rog
Exact steps for others to reproduce the error
1- Create a UV sphere
2- Deform some points on the left side (+x, doesn't matter which side really)
3- Invoke the "Snap to Symmetry" and set the symmetry direction to +X to -X
4- No matter what you set with the threshold value or the Factor, the deformation on the left side will not be translated to the right fully and/or correctly.