The workflow for each Task SLA record maintains its own set of values for determining how much time has elapsed. Workflow uses these values to trigger the notifications to users when an SLA has been in progress for the percentages specified in the SLA Percentage Timer workflow activities.
Workflow maintains its own calculation deliberately so it is not dependent on the values stored in the Task SLA record as these may not be up to date.
There is a known error for this issue which includes a workaround that can be viewed at KB0563889.
Note: This issue has been fixed in Helsinki and later releases. The system now updates the Task SLA record whenever an SLA Percentage Timer activity in the workflow expires.