Why is 2 / -2 / 2 equal to -2 in OpenSCAD? (Mathematical Order of Operations)
I suspect the behavior you are seeing is an undocumented feature (aka, bug) of OpenSCAD. I've found in the latest stable release that if the - is placed on either end, the result is -0.5, but in the middle, my results are the same as yours. Surrounding the -2 with parentheses results in a correct answer, however.

It appears that the parentheses turns a mathematical operation into a signed integer. It follows that the operations without the parentheses is right to left:

2/2 = 1, negative 1 with the minus, 2/-1 = -2