First off, grats on some awesome results! 99% is awesome! The reason Subt doesn't result in thousands of errors when done incorrectly is that when you scan one DPCI incorrectly it is only logging one single error, not 9999 of them because eaches are not what is measured in that metric. One error out of around 20,000 scans in the back room through the course of a week doesn't have any impact on your score. It is entirely possible that Subt doesn't get factored into BRLA but it shouldn't be impacting it significantly one way or the other because most stores don't use that function more than a few dozen times a week.
More evidence for MisterLogistics here. If errors were based on eaches how would the following work? A case pack of 24 bottles of shampoo is baffled in your uppercase location. When you go through a pull looking for something else you scan the pick label on the shampoo. If errors were based on eaches, the system would have to stop you to ask how many are in that casepack or else it couldn't possible ding you for the appropriate 24 errors. Therefore, the system dings you one error for having one scan that was an error, not 24 errors because it doesn't have that information.