* Day to day there was wild variation in diet ... at the end of the week the average daily calorie intake was very consistent/predictable

I think this is quite normal and why people recommend not measuring yourself every day. The average person's daily calorie burn is generally not as stable as they think it is unless they are closely following a plan.

You can calculate base rate then add the standard percentage for a normal relatively sedentary lifestyle to guestimate your normal daily output but that ignores the fact that you body's daily routine is not fixed. In your working day you will walk around more or less some days, the difference won't be massive but if you are micromanaging (which you probably shouldn't: that can lead to over-worrying and potentially to certain physiological eating disorders) it counts, your social hours will vary too and probably more so on a day-to-day basis then your working and "doing nothing" hours, your environment will vary, particularly by temperature which will affect what your body does to maintain internal equilibrium, and unless you eat exactly the same food at exactly the same time every day there are differences there: different foods take different amounts of time to digest, have their energy extracted, and have their waste pass on through, have different water content which adds to the natural variation in what you drink, different amounts of drugs (caffeine and alcohol for most people) that affect water intake/output in the body is an issue and so forth. There are extra complications for a woman: changing hormone levels over the course of their regular reproductive cycle can make quite a difference to water retention and hunger levels.

Basically: your body's routine does not follow an exact 24 hour cycle - this is why your weekly diet is a lot more predictable than your daily one, over the longer time the natural variances average out.

But we do have mechanisms that alter out hunger levels to try make sure things do even out over time. There are things that these mechanisms don't account for though: problems such as diabetes and other illnesses that affect energy absorption and use, and it is generally believed that a high sugar diet "breaks" these systems because they are geared towards slow-ish release foods and the energy in compact food high in small sugars is very quickly either used or stored away (so you have the energy ready for use, but you become hungry because the food is so easy to break down that it has already passed the parts that register hunger).

I monitor my intake very closely, having gone (over the last ~30 months) from ~18.5 stone to ~10 and wanting to stay somewhere near there. I weigh myself every day or two but only really watch a fortnightly average as a key value because I know there will be daily variations both that are due to either my routine not being very routine or sometimes are far less intuitively understandable.

It is actually easier for athletes because they can measure their extra calorie output with some accuracy, and if you are training a lot then this output is going to be more significant than the random daily variations. Their incoming diet is likely to be far more predictable than that of the man on the street too. I'm not an athlete by anyone's definition but I do exercise "recreationally" these days (mostly running, some strength/tone work, some cycling & walking for fun/exercise not just to suit commuting requirements) and I do notice the results being more predictable in periods when I do quite a bit of that compared to times when I do very little.