Car batteries are also made to a standard spec and are off the shelf parts that are not exclusive to any particular brand or car.
The thing with the 'official' parts is that if they are genuine Apple parts they would have leaked out of the supply chain somehow. Genuine parts are only available directly from Apple or an Authorised Apple seller. Lets not kid ourselves this is fraud or theft and not something we should be supporting.
I also cant believe that the batteries are made in the same factory, to the same spec as the Apple ones and sold to the grey market. This isn't some poorly managed public sector contract, Apple will have exclusivity on their design and specification. The company producing the batteries will be contractually obliged to not do this and if this was happening they would not be producing apple products any more.
It's more likely that the they have reversed engineered the genuine battery and made to a similar specification by a 3rd party. There is only so many ways these are made and are similar year to year so it wouldn't be that hard to do.
Anyone selling 'genuine' apple batteries are either a) committing fraud themselves by passing off something that isn't genuine as genuine or b) someone in their supply chain is committing the said criminal act. I can't honestly believe that anyone in the phone repair industry doesn't already know this.