Why are my SKAN reinstall numbers so much higher than Branch usually shows?

SKAdNetwork reports reinstalls via the redownload postback parameter, which Apple defines this way:

A Boolean flag that indicates the customer redownloaded and reinstalled the app when the value is true.

Similar to the Redownload numbers provided by Apple Search Ads, this data is likely coming directly from Apple's internal records for that customer. That means it is different from (and more accurate than) anything a third-party like Branch could infer, even prior to iOS 14: depending on the situation, Branch would see these events as either re-opens or simple installs.