Suppose I have a glass full of water, upside down, so that the open end is at the bottom, resting on top of a weight measuring scale. Now from what I've learnt scales show the weight as the external force acting upon it excluding the reaction force from the ground(or so my professor told me while I was solving those elevator problems). So if I put the glass full of water like I said, inverted on top of the scale, shouldn't the hydrostatic pressure of the water exert some force on the scale too(just like a did exerts pressure on the walls of the container)? Wouldn't that cause the measured weight of glass+water to be higher than the actual weight?
Weight of water causes pressure. The scale measures weight (force), not weight-per-unit-area (pressure), and makes the correct summation. To confuse the scale, one could rest a thumb on the pan, or attach a string and lift: there would then be a force OTHER than weight and the structural support of the scale pan that counters the weight.