Maxperson
Morkus from Orkus
I think if it was intended to be a measuring stick, they'd have listed a larger range of Adventuring Day XP numbers, rather than a single total for each level. They'd have gone with something like...I would say the problem is just the phrasing "XP that character is expected to earn in a day". I think pointing out the guideline of "here is how much to expect characters can take in a day" is a great guideline for designing encounters. This makes it easier to consciously design encounters across an adventuring day to be deadly, likely to kill some but not all characters, pushing their limits but doable, engaged but not really risky, or easy combat. D&D should be designed for a lot of different pacing and challenges here, not just expect to push them to the average design limits every day.
Take out that phrasing in that one sentence and I believe it is a measuring stick for challenge per long rest, not an expectation.
Character Level: 1st
Easy: 100
Medium: 200
Hard: 300
Deadly: 400
TPK: 500+
And so on. Instead they simply say that the expected amount of experience per PC is 300, with the guidance to spread that out over 6-8 medium to hard encounters, less or more if there are easy or deadly encounters, and then let the DM figure it out from there.