This is really weird but for all the years I've been using SolidWorks, this is the first time I've noticed this: when thickening a surface and using the midplane option (Thicken both sides) the value you enter is applied to both sides. Therefore the total thickness is actually 2X of the value you enter - i.e. thickness value is applied to both sides.
Does this seem right to you or you really don't care? I'd like to hear from you. It doesn't seem right to me that the parameter value for this feature, when using this option is actually 1/2 the total thickness.