Looking on the wiki, it feels like some skilling guides don't take into account the opportunity cost of grinding other skills or grinding for gp.

It’s ironic that you go on to use fletching as your example because this is the first paragraph of the Ironman fletching guide lol.

[https://oldschool.runescape.wiki/w/Ironman\_Guide/Fletching](Fletching is most often trained through making broad arrows, as the ability to purchase all the ingredients to make them cuts out any frustrating preparation time (at the cost of coins to buy the ingredients). The stackability of all ingredients in making broad arrows makes it an easy skill to train in tandem with other skills, such as Runecraft, Agility, Farming, and more.)