Those 15 minutes are meant to accommodate for time waiting in the
lightning lane, though, which more often is exceeding a 15 minute wait (and DAS users who can't wait in it for reasons that require the DAS in the first place are then having to get out of line and forfeit all that time waited in the first place)
To add, there's quite a few times DAS users wait
more than standby guests. It's not uncommon to see people saying a ride said it had a 40 minute wait time, for example, but they only waited 12. DAS users can't have that experience though, they have to go by what the standby time says.