Since when is a size 14 three different sizes? Disappointed by Woolworths

Since when does size 14 mean that it can be any size. I visited a Woolworths because I wanted to get myself a few, what I thought would be, high-quality clothes for work. I too a navy blue pair of pants (nr14) to the fitting room. It fit perfectly, so I went back and took the grey and beige pair of pants as well. The grey and beige pants were exactly the SAME make and cut as the blue pair and I took both in the size 14. I did not fit them, thinking Woolworths is supposed to be quality clothing. The pants were R450 each. I paid cash. I got home and our domestic worker removed the tags and put the pants in my cupboard. When I tried on the beige pair they were at least a size too big. I did not wash the beige or grey pair, as they were too big and I could not wear them. I went back to find out if I can exchange them for a smaller size. I took the blue pair along to show them that the size 14 differed for each one of the pairs. They did not want to exchange it as they said I washed them (they smelled like my clothes because they were in my cupboard together with my other clothing) and they did not want to believe me when I tried to show them that the sizes differed even though they are all marked 14.

Now I know that I made a mistake in not fitting them all before purchasing. So I made the mistake of trusting Woolworths. Now I am stuck with R900 worth of clothing because a retailer cannot provide the quality they promise. I am a student on a student scholarship and R900 does go a long way. I am really disappointed in Woolworths and will think before I shop there again.