Official comment
Tom,
You are correct. Actual Start Date does not have a bearing on Resource Load. It is based on Start and Due dates as well as overall Duration and Work. This is broken up by days throughout the duration of the task based on the resource availability (unit%). As an example, 100% (unit%) for a one day task. The resource would be available for 8h of work per day assuming that their calendar is specified for 8h of working hours per day (some are configured differently but the default is 8h).