I'm not privy to anything specifically about batteries, and you may be right about the batteries all effectively being the same.
However, I do know - in general - component manufacturers are notorious for cutting corners to save a few cents, doing additional factory runs after hours, altering chemistry, faking specs, etc. It's a shady business climate that needs constant QC.
Also a fair amount of what you pay for is a known chain of custody from component manufacturer to your device. From what I can tell for many components there isn't a clear feedback loop to end customers, as parts travel through several intermediaries, which means its less likely bad quality will lead to repercussions to the component manufacturer. It goes all the way to the repair place, who's typically doing a one-off order for a customer rather than having an ongoing customer relationship, and who may never see a failure 6 months down the line.
As far as batteries, and this may be a hypothetical as I do not know about who's manufacturing what and at what quality, but there's also legitimate safety concerns which is why a higher than typical level of scrutiny is required.
These are all conditions that could lead to potentially higher failure rates. How much higher is debatable, it may be only a percent or two, and for many folks a small additional risk may be worth saving some cash for.
Personally? For older devices I wouldn't care. A newer device, I would.