It's mainly just a refutation of the standard assumed hireling costs - 1sp/day for an untrained hireling[1], 1sp/day for "poor" meals[2], and taxes/tithes of up to 20%[3] are three numbers that really shouldn't coexist.
That the DMG tries to justify it[4] somehow just calls attention to it and makes it worse, really.
The only way I can see to make everything go together is the good old AD&D 1E explanation:
In other words, literal goldrush prices.Originally Posted by 1E PHB p.35
...In case you wonder, yes, the 1sp/day price is straight out of the 1E DMG - bearers and torchbearers cost that much. A "merchant's meal" is also 1sp, but that's not exactly what you're linkboy is getting (which is presumably cheap enough that your thousands-of-gold-pieces PCs don't need to know the prices - their rations are bought for 50sp a week!).
This is a case where the added detail just makes older ad-hoc prices look really weird.
[1] PHB p.132: "The amount shown is the typical daily wage for laborers, porters, cooks, maids, and other menial workers." (The DMG p.105 also includes barristers in this price class, and mercenaries and valets/lackeys both sit at 2sp/day - between trained and untrained price classes.)
[2] PHB p.131: "Poor meals might be composed of bread, baked turnips, onions, and water." (PHB p.129 gives bread as being 2cp/loaf - turnips and onions aren't even in the Arms & Equipment Guide, however.)
[3] DMG p.140: "Taxes paid to the queen, the emperor, or the local baroness might consume as much as one-fifth of a character’s wealth[.]" (The same page also gives tithes as being 10%, of course, but those are called out as optional except in tyrannic theocracies. 20% is also given as an upper limit on taxes.)
[4] DMG p.139:
The economic system in the D&D game is based on the silver piece (sp). A common laborer earns 1 sp a day. That’s just enough to allow his family to survive, assuming that this income is supplemented with food his family grows to eat, homemade clothing, and a reliance on self-sufficiency for most tasks (personal grooming, health, animal tending, and so on).