i would assume when talking in percentiles it is always going to be relative to what amount you have, if they were talking in terms. of starting budget that would be insane.
for example week 1 a sleeper wr pick up 5% of your budget is ($100faab) is $5 week 8 a sleeper wr pick up 5% of your budget is ($20 of your $100faab left) is $1 because 5% of ur original budget would be equal to 25% of your current budget in the situation i described.
all in all the faab % imo are more of a "i like this player a lot" vs "this is someone to think about if u have a roster spot" scale to help waiver wire articles have some sense of a ranking scale of not just who you should pick up in a particular order for that week, but also how important/big of a pick up it is in comparison to all pick ups throughout the year.
good rule of thumb that i use for my survey (your roster decisions crowd sourced) section on waiver wire adds 5% or less = more of a sleeper/stash 10-15% is someone tht you will prob be competing with other owners to get
25% or more is probably some. one tht will be a starter and your entire league will be after them (maybe 4-5 of these all year major adds due to injs to star players)