Or that.
You see, elves regard animals as low-value, and cloth as high-value. If they like you, they will bring more cloth. If not, more animals.
This isn't quite as true anymore.
In 40d, pack animals' carrying capacity was quite limited, binned materials (like cloth) was always loaded first, and if there wasn't enough weight to fit everything a caravan wanted to bring, they'd simply not bring the rest. So, if elves were happy enough with you, they'd load up their bins and bins of cloth, and have no carrying capacity left for all of the animals that they wanted to bring, so they didn't. If they didn't like you, on the other hand, they'd try to bring less stuff, so they'd have room for non-binned things like animal cages.
In 31.x, however, animals' carrying capacity was massively buffed (perhaps to compensate for the missing wagons), so you will practically never see an elven caravan that is completely filled with cloth. They simply can't become happy enough with you to bring that much cloth. So, now, if the elves like you, they'll bring more of
everything, including worthless cloth and wooden armor, but also including precious exotic beasts.
I've also seen some people claim, and hints from ingame behavior, that traders will tend to bring more of things that you've bought before. So, if you buy animals whenever they come, the traders will be more likely to bring more animals. This also means that if you buy-with-a-100%-discount all of traders' cloth, they'll also start bringing more of that. As a result, it's probably better anymore just to treat elves more kindly than you might treat, say, goblin lepers, as they're not completely worthless anymore.