Difference between Enhanced and PTP batch is the 2 months of lesser service as NSF.
Enhanced applied if you pass IPPT. This gives your NS duration of only 1yrs 8 months.
PTP batch is for those who fails, and NS duration will be 2 year, due to the additional time for train your fitness. In another words, MINDEF wants all it's pre-enlistees to be fit and reward you as an incentive.
Enlistment period is different from Enhanced and PTP. The "enrollment" (if this term is more familiar to you as I believe is the term used for entering JC or Poly) for Enhanced and PTP will usually be of a different date or months.
For your info, refer to:
https://www.cmpb.gov.sg/web/portal/cmpb/home/before-ns/enlistment-dates
The above link shows the past and upcoming enlistment schedule. You can see some has "(PTP)" indicated. So those entering NS on the dates that has PTP will only ORD exactly 2 years later. Otherwise, 1yrs 8 months.
Hope this answer your query.