Why did the Mars team drag 56 days of food into the Hab for a 31 day mission?

Question

In this answer trying to explain what it meant that the Hab was designed for a 31-day mission Valorum points out the book says:

The surface mission was supposed to be thirty-one days. For redundancy, the supply probes had enough food to last the whole crew fifty-six days. That way if one or two probes had problems, we’d still have enough food to complete the mission.

Ok. 31-day mission, 56 days of food in a bunch of supply probes. Got it.

But given how valuable time is on the surface of Mars, and what a pain in the neck it is to carry crates of crap around on Mars while wearing their suits and going in and out of the airlocks, why did the crew drag all 51 days of food back into the Hab instead of stopping after getting their 31 days? Or maybe 32 or 33, in case anyone got extra hungry after a meal or two?

(Extra credit: So ... in the movie, where were those landed supply ships anyway? The crew surely didn't bury them like it did the RTG. If they're all out of sight from the Hab, well ... they really didn't want to drag 51 days of food back to the Hab.)

Answer 1

The extra food is there to act as a contingency in case one of the hazards of the mission to Mars means the crew will need extra food supplies.

There are a number of possible reasons why they might need extra food some of which I will suggest here, but others might come up with other good reasons. Accidental spoilage a particular box of food might be damaged in landing or the trip to Mars. This might be in a manner that is undiscovered until the box is opened when it's needed. It is a good idea to have all the supplies stored away in case of this.

It's also a reason why mixed ration boxes are a good idea. If you pack boxes by type you could lose all your supply of beef stew in one fell swoop.

Another hazard might be Martian weather of a lower magnitude than the storm that forces the crew to evacuate but still bad enough to make EVAs unviable. Those extra boxes of food are no use if you cannot reach them.

Answer 2

If we take the film at face value, the simplest explanation is that each food box contains a selection of different meals. There isn't just a single box marked "mac and cheese" and another box marked "beef stew", etc.

Bringing all the boxes back makes sense if you assume that each of the crew want to have a choice of meals rather than having to accept a random (or whatever system they use) choice determined by limited availability.