It's possible that they made a run of chain with all the cutters leaned over on one side or something, but I've never seen that.
My guess is that you're using the kind of raker gauge that lays on top of a tooth and subtracts 0.025.
Here's a more complete view of why they say you should set all the teeth and rakers the same to cut straight. The devil is in the details.... and it turns out that you can cut crooked with all the rakers the same height, and you can cut straight with different length teeth.
What makes a chain cut crooked?
One side grabs more than the other.
What makes a tooth grab? Top plate angle, hook angle, and the angle that the wood hits the cutter at (which is determined by the raker and the saw operator).
Could be the bar, but you ruled that out. So the chain!
Almost nobody hand-files both sides at the same angle or to the same size. My neighbor says he is an expert lifetime hand filer, and his actual angles and tooth sizes are terrible. I know this because his chains come to me for fixing. This isn't criticism of him- this is typical of a used chain once you look at things with a micrometer and an angle gauge.
Anybody who says they are a great hand filer and also says sometimes you just need to take a chain in to be sharpened- they are struggling to admit the truth to themselves. Almost everybody is laughably bad at eyeballing this stuff. It took me 30 years to admit that I was terrible at hand filing- but once you bite the bullet and start measuring your results with a micrometer, you are forced to admit certain truths.
Teeth lean, chains lean on the bar, and the same size tooth looks a lot different from the front and the back sides. People are bad at telling whether left side cutters and right side cutters are the same size and whether rakers are the same height via eyeball.
They're also bad at telling whether the top plate angle is the same on both sides, especially if the chain has no witness marks. This can make you go crooked all by itself, even if all your sizes and heights are perfect. (Although
@Philbert has a great photo in another thread... if you twist the chain a little and hold a left side cutter exactly side to side with a right side cutter, the Mark I eyeball gets a lot better at telling what's going on.)
Rakers have the same issue we just discussed, and are critical to determining how much grab its tooth has. But rakers are even trickier than cutters if you are working via eyeball, because it's the angle between the raker and its cutter and the wood that matters- not the height of the raker.
So your typical hand filer will normally have this side teeth shorter than that side teeth. Then they take a raker gauge that sits on the tooth and subtracts 0.025" (now we're cutting crooked because the short tooth is catching air while the tall tooth is grabby and overstuffed with wood). Or they use a raker gauge that lays across two opposite side teeth and subtracts 0.025" (this is better but still crooked). Or they use their magical eyeball and pick a height that looks right (anything could happen with this one).
Take a crooked cutting chain with perfect top plate angles but different size teeth on either side. Perfectly set all the rakers to 0.025" below the height of each tooth. It will still cut crooked, because the long tooth grabs more than the short tooth.
Now take that same chain and set all the rakers to about a 6° AoA for each tooth. It'll look funny, but it'll cut straight again (remember, the top plate angles are the same on both sides for this chain), because each tooth is taking the same size bite of wood as it comes around.
So the conventional wisdom for this situation would be to go get a pair of calipers and a file guide, find the shortest tooth, file all teeth back to that length and fix the top plate angles while you are at it, and then set all rakers to .025" below tooth height. (And that will make it cut straight.)
Alternatively, you can just fix top plate angles and adjust raker AoA and get a funny looking but straight cutting chain.
