If you are doing weight lifting that can make your thighs and sometimes even your waist bigger, if you are doing squats especially.
Also, not only does the size not matter, when they cut out the fabric they do it in multiple layers. The more inexpensive the clothing, the more layers of fabric they try to cut at once. The layers of fabric cause there to be small differences in the size of the resulting pieces, enough so that two pants that are both the same size can feel almost a whole size different from each other. I've got some pants like that. Exactly the same pants, same size, but one is super loose.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 187.5lbs, press 75lbs and deadlift 200lbs