A trading value is a summary of what people actually accepted, expressed on a scale that lets unrelated items be compared. It is not set by the game, it is not a price, and no site has an official version of it.
Completed trades beat opinions
The only evidence worth anything is a trade that went through. Offers do not count, because anyone can offer anything. A value moves when several independent completed trades stop matching the listed number.
Why two sites disagree
Different sites weight demand differently, update on different schedules, and some simply have not been touched in months. If a list has no change log, you have no way of knowing which of those applies.
Spotting a stale list
Look for three things: a visible last-updated date per item, a public change log, and demand ratings that differ from item to item. A list where every item shares the same demand number is not being maintained.