SLA in Service Catalog

Dear all

For SLA In Service Catalog we have to set it separately from Incident.
But there is a question that I have!
For example when I set the SLA for one template , in workflow I set the SLA to Low.
When that requests arises and the priority of request on that time is Medium,if the technician changes the priority from Low to Medium the response time does not change.just the status changes to Medium.why ?

What is the difference between the SLA in workflow and priority in each template.
As I understood correctly preference of SLA in workflow is higher than SLA which sets in priority of template or a technician changes.

Any Idea?

Regards
Negin