This is patently incorrect behavior so I'm surprised I don't see anyone else reporting it.
Set an SLA timer so that tickets w Priority High must be resolved in 1 hour.
Take an old (> 1 hour) ticket whose priority is Low - set it to High.
SLA alert immediately fires.
SLA's must be set from the time the Priority changes, not from the time the ticket was opened!
If no one else sees this perhaps there's something odd with my setup.