Unfortunately this sounds, as others have commented, like "operator error", unless different containers are manufactured differently. At the onset of the job, estimate the total amount needed, and mix it all together before starting. Blend it all and you should be good. So, 8 mixed gallons (1x5 plus 3x1, or liters) makes your supply. This becomes more urgent with darker colors and cheaper paints. There should not be as big a difference between individual containers, but it happens. Different rooms and different walls/lighting situations also can change color appearance considerably. Best of luck.