Maybe a counter-example will help illustrate: assume Minion is better than Lab. Now Horse Traders comes out. Horse Traders actually changes the value of Minion in absolute terms. Even though we won't often have Minion and Horse Trader in the same kingdom, it at least makes sense that the existence of cards like Horse Traders would change our valuation of Minion, not relative only to Horse Traders but relative to everything. It is thus a plausible argument that the creation of Horse Traders (a $4 card) could change the ranking of Minion and Lab ($5 cards) relative to each other.
I randomly thought this post was very instructive.
Here tlloyd notes that "Horse Traders actually changes the value of Minion in absolute terms." It does so because, when Horse Traders is on the board, Minion becomes weaker, where weaker means that it's a worse buy.
If this is in fact true, then the existence of Warehouse does decrease the value of cellar in absolute terms. This is because in just as frequent a situation (i.e. where Warehouse and Cellar are both available) Cellar is a weaker card. It's weaker because in those situations Warehouse is also available. Thus the very existence of Warehouse actually does decrease the absolute value of Cellar, the same way the existence of Hunting Party actually does decrease the absolute value of Lab. I'll also note that the hypothetical diamond card would decrease the value of Gold...it wouldn't make Gold a bad card, because Gold is a very good card. It would just make it slightly worse, which would still be very good.
This happens because we play with cards in sets of 10, therefore every time new cards are created, it changes the value of other cards. How many times have people noted that Hinterlands has "made several previously underpowered cards stronger"? This happens because the situations in which those cards are useful have become more (or less) frequent.
So, Warehouse did make Cellar a worse card, because we are considering all possible boards and some of those boards do have Cellar and Warehouse. If Seaside did not have Warehouse, then there would be more situations where Cellar would be useful.
Anyways, I'm just trying to say, that Theory's old wording was fine. His new wording I think is better, because it's broader, but to argue that Warehouse had no impact on the absolute value of Cellar is just silly, maybe a small impact, but it did have an impact.
All of this happens because the only meaningful way to rank cards in Dominion is to rank them across all possible layouts.
While there's no formula or anything going on, it's just based on experience and feelings. based on the paragraphs, and various explanations given by others there seem to be a couple factors which go into ranking the cards.
1. How often do I want to purchase the card (this is equal to how often is the card worth the opportunity cost to purchase which is very similar to how often will this card help my deck)
2. How much help does it give me when I get it
The second piece above is easy to understand, and I think most people agree.
The first piece above seems to generate lots of misunderstanding...people saying for instance "you can't judge a card based on how bad it would be in your deck...you shouldn't be purchasing it in that case". Well, how do I determine if I should purchase a card? If it would actively harm my deck, then I shouldn't buy it. That means that it isn't a good card (of course when I say "me" I mean someone who's very good at the game, who could make these judgments)