If there is a reason why going through official repair channels is more expensive than third-party, it is because they have been authorized by the company who made them, versus third-party repairs who might turn to similar but unauthorized parts. However it seems that even purchasing directly from the company could be a little dodgy sometimes.
According to a report from DigiTimes (via Cult of Mac), it seems that Apple manufacturer Wistron has been accused of allegedly using unauthorized parts in the manufacturing of the iPhone 8 Plus. Note that the report only points to one of Wistron’s plants in Kunshan, China, where production has been allegedly suspended for two weeks.
However Wistron has denied the reports and claims that “operations remain completely normal”, although the reports have also suggested that Wistron has penalized several mid and high-level executives who presumably might have been complicit or who might have missed the problem. Like we said, given that the issue seems to be limited to just one plant, we doubt that it is a widespread problem.
The parts in question are apparently related to waterproof components, which means that there is a potential that their waterproofing might not be as secure, or at least not up to Apple’s standards (it could be better for all we know).