Perhaps it is carbon dioxide. Here is a paper on it:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3341709/
To summarize, the idea is that high partial pressure of CO2 causes blood pH to change, which influences the body’s regulatory mechanisms and eventually leads to obesity.
At higher altitudes the carbon dioxide fraction in the air is unchanged, but the partial pressure is lower. I’d expect that lower partial pressures of CO2 would mean less effect on blood pH.
I made an account for your challenge.
I plotted items by color and looked at minimums, maximums, and averages. Yellow items consistently provide just under twenty mana while blue items always provide about the same as the reading, except that they sometimes provide twenty extra. I was too lazy to try to figure out the red or green items. Given what I know, I can submit the blue items HoC, RoJ, and PoH to get about 140 mana for 101 gold, and call it a day.
However, I also noticed that no blue item provided over 60 mana. I will add in the yellow items PoP and WoJ for a margin of safety. Painfully, that brings the total to 177. Combined with the chance that other items will provide extra it should be enough for some confidence.
The Wizard Wakalix did not inform me of the reliability of blue items; they merely called their device a liar. (Perhaps they aren’t yet aware of the pattern because they don’t utilize the arcane magic known as python). I may as well include a letter that outlines my findings regarding yellow and blue items. That way, the next time Wakalix goes to the caravans they can make use of the information, and the next time they want to hire someone I will be their first thought.
If I were devious, I might try to include red and green items to obfuscate the reliability of blue for predicting mana cost. Sending three blue items at once will surely let the cat out of the bag. However, If I expected deviousness, this setup is an excellent way to deceive an errand-runner into providing free magic items. I’ll count on the norms regarding my work to prevent either of those outcomes. I may check with other errand runners to make sure Wakalix isn’t running a scam, just in case.
Edit: Reading the comments allowed me to notice I mixed up the direction of the error: blue items read 20 over, not provide 20 over. Well. Good thing I included a margin.