iPhone users get free original quality backups because the iPhone itself saves its images in HEIC format according to Android Police. The format provides a smaller file size than a compressed JPEG that most Android devices use. Since images in HEIC from the iPhone were already in a smaller file size than Google devices offer, they weren't compressed further from their original size to the "high quality" setting.
Google has confirmed that images captured in HEIC/HEIF formats aren't compressed and aren't charged against a Google Photos users' quota. Google has confirmed that it is aware of the issue and is working to fix it. How exactly Google plans to fix this issue is a mystery; it's not clear if Google will start charging for HEIC images stored in Photos even though the files are already small. Google could also take the unimaginable step of using additional resources to convert the images and compress them further. Either option doesn't really make much sense, so it will be interesting to see how the company steps over this minefield.
Apple isn't the only smartphone maker that uses HEIC, Samsung devices also support the feature, but the format isn't widely used with that segment of smartphone users. The last time we talked about Google Photos was when it received an update in August 2019, allowing it to search for text within photos.