Paradigm "Contact Start" and "True to Interval" as it applies to email activity

Can anyone help me understand the different usage of the Paradigm "Contact Start" and "True to Interval" as it applies to figuring out how many emails come in on any specific day? I pull the skill performance with Contact start and get 232 queued for one email skill. When I pull it with True to Interval I get 525 queued for that same skill.