The Torah tells us that Dasan v'Aviram saved over mon for the next day. They didn't receive more than an omer per head, like everyone else. It was impossible to take more or less, so their solution was to eat less today so some mon would be left over for tomorrow. It was important to them so they could be assured that there would be food for themselves and their families tomorrow.
However, the Torah tells us that their plan failed. (16:20) ויבאש תולעים וירם, "[The mon] became infested with warms and it stank..." It didn't help them to save for the next day. Whatever was left over was spoiled. They didn't gain anything. They only lost because on the previous day, they ate less, so they could leave over for the next day, and whatever they left over spoiled.
But they lost even more than that. The next pasuk (16:21) states, איש בבקר בבקר אתו וילקטו אכלו כפי, "They gathered it morning by morning, each man according to what he eats..." The simple translation of this pasuk refers to Klal Yisrael, telling us that they would gather the mon each day. However, the Malbim says that just as pasuk (16:20) refers to Dasan v'Aviram, telling us that they left
