ETC does not get decremented on tasks when actuals are newly posted on an adjusted timesheet
Adjusting a timesheet will not further adjust ETC on a task assignment. The reason for this is because when you post a timesheet for a given time period, the time period's finish date is used to set the 'actThru' for the assignment. Setting the 'actThru' on the assignment is primarily what decrements the ETC.
There is also another feature of posting time that is more subtle. When you post time for a given resource, ANY assignment that resource is assigned to gets the 'actThru' adjusted, regardless of whether or not that task assignment is actually on the timesheet!
Now, here is the subtle part. On project B (no time posted originally), the 'actThru' would have been adjusted forward due to posting against project A, BUT THE ETC WOULD NOT HAVE DECREMENTED (since no time was posted). It would have been pushed forward unless the loading pattern on the assignment was 'Fixed'. You will have noted an increase in the finish of the task assignment on project B after the initial posting. It will have increased by one day and the ETC from the period posted will have been pushed into that new day.
Now, when you go back and adjust the posting, posting 'new' time on project B, the 'actThru' is already set and no further ETC decrement will be seen.