What causes weight gain on the scale...

1) eating above maintenence
2) hormonal fluctuations, including TOM for women
3) Intramuscular fluid retention from increased intensity of exercise
4) Excess sodium in diet, too low potassium:sodium ratio
5) Constipation
6) Eating/drinking prior to weighing
7) Chronic dehydration (counter-intuitive!)
8) Illness/Disease causing swelling
9) Water retention as a result of increased muscle glycogen storage from carb-loading

I'm sure there's more I'm not thinking of. Point is, many many things can cause a weight gain.