It's been a long time since a looked at it in any detail, but a quick search for "calories burned sedentary" gave me a page full of calculators and estimates.
Looking quickly at a few of them, they mostly have tdee estimates for my age now that were still to high for me at the peak of my exercise fitness a decade ago... That includes the calculators trying to take weight and height into account.
The issue is that most of these are repeating and deriving categories based on guidelines set at points in time when a much larger proportion of the population had jobs that involved physical exertion.
And so you can be much more active than average for todays population and still be considered sedentary relative to the activity levels assumed in guidelines.
E.g. the UK NHS guideline of 2500 kcal for an average male would take me adding a couple of hours of walking during the day to reach despite exercising more than average...
And I'm sure you're right that people's failure to estimate their activity when they are exercising or being active is also a big part of it.
Looking quickly at a few of them, they mostly have tdee estimates for my age now that were still to high for me at the peak of my exercise fitness a decade ago... That includes the calculators trying to take weight and height into account.
The issue is that most of these are repeating and deriving categories based on guidelines set at points in time when a much larger proportion of the population had jobs that involved physical exertion.
And so you can be much more active than average for todays population and still be considered sedentary relative to the activity levels assumed in guidelines.
E.g. the UK NHS guideline of 2500 kcal for an average male would take me adding a couple of hours of walking during the day to reach despite exercising more than average...
And I'm sure you're right that people's failure to estimate their activity when they are exercising or being active is also a big part of it.