Estimated hours left
Estimated hours left (HL) is the value assigned to the 'Hours left' field and equal to the Estimated hours value in the corresponding field at the time an activity is created. As long as there are no actual hours recorded for the activity, the two parameters are identical, i.e. changing either will change the other automatically. This is normal behavior when the activity is in its planning phase, when you are in the process of determining how many hours to allocate to it.
Actual hours left
As soon as an assignee enters a time log, which can be treated as the moment of actual work started on the task by that assignee, his or her personal hours left value changes accordingly (see Adding time logs for details). The 'actualization' of personal hours left results in the broken link between hours left and estimated hours. Henceforth changing estimated hours will have no effect on hours left, and vice versa. This is normal since as soon as there are some actual hours recorded for the task it is considered as started or past planning, and estimated hours cease to make a difference any longer having been fully substituted by the hours left value.
1 The one exception to this rule is unassignment of an assignee. When an assignee is removed from the team, both their estimated hours and hours left are deducted from total EH and HL.