Prestart hours left
Prestart hours left (HL) is the value assigned to the 'Hours left' field and equal to the Estimated hours (EH) value in the corresponding field BEFORE the activity is started. As long as there are no actual hours recorded for the activity (which signifies the start of the activity), the two parameters are synced, i.e. changing EH will change HL automatically. This is normal behavior when the activity is in its planning phase, i.e. when you are in the process of determining how many hours to allocate to it.
Hours left
As soon as an assignee enters a time log, which can be treated as the moment of actual work started on the activity by that assignee, his or her personal hours left (PHL) value changes accordingly (see Adding time logs for details). The 'actualization' of PHL results in the broken link between HL and EH. Henceforth changing estimated hours will have no effect on hours left[1]. This is normal behavior 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.