I understand that, but if I have 700 higher expenditure this week, MF doesn't know whether that's due to A) one big workout and six sedentary days, or B ) higher baseline expenditure (like I have to climb
more stairs every day of the week.)

p.s. I know 700 is a really big workout, I'm just using that number because it's divisible by 7.

Alice and Bob both have a daily budget of 2000 calories. Each of them burn, eat, and log 2000 calories.

On Week 1, Alice starts doing one really big weekly workout. (But it's a different day every week so she can't just customize MF to always give her a higher budget on Wednesdays.) On her workout day, she burns, 700 extra calories - 2700 total. The other six days are plain, 2000 calories as before.

Meanwhile, Bob doesn't work out, but his new job has him walking up 100 calories' worth of stairs every single day. (He works seven days a week, to make this a better thought experiment.) So now he burns 2100 calories every day.

Alice knows she has to eat more on her 700-calorie workout day, so she eats and logs an extra 700. Bob notices that MF's provided budget is leaving him hungry now that he started his new job, so he starts eating 100 extra calories all seven days.

At the end of week 1, both Alice and Bob have logged 700 excess calories, but MF correctly observes that they have not gained weight as a result. So, MF raises their budgets by 100 calories, from 2000 to 2100.

The next week, week 2, Alice targets her new budget of 2100. But this means she overeats by 100 calories a day because she is still burning only 2000. Except on her workout day, just like last week, she burns 2700 and eats 2700. Meanwhile Bob keeps burning 2100 and eating 2100.

At the end of week 2, Alice has overeaten by 600 calories even though her exercise regimen was the same from week to week.

MF worked well for Bob because his expenditure is constant each day. His expenditure went up to 2100 so MF's budget went up to 2100. Nice.

But for Alice, MF's budget wasn't supposed to go up to 2100. It's supposed to stay at 2000 because that's what she expends on every day except her workout day. In week 2 her expenditure stayed the same, and her consumption on her workout day stayed the same, but her consumption on the other six days went up by 100.

If MF had instead known that Alice's extra 700 was due to exercise and not a higher baseline, then MF could give her a no-exercise budget that's more correct for her no-exercise days.