Sometimes we do not focus on small things. In this case it is just a small thing to focus on.
Before entering the global variable you must enter the equal sign to assign a global variable to the dimension. If you do not enter the equal sign, you can create a new global variable, but it will not be assigned to the dimension.
That is the only reason why solidworks do not change the dimensions in part as changing equations.
Because not using equal sign means technically we are not linking the global variable to the equation. This is just a kind of learning that how these small things plays an important role.