When you're listing against an existing ASIN (an ASIN that you did not create) it is uncertain whether Amazon will incorporate your data on the listing. Priority is given to the seller that created the ASIN. The number of sellers currently listed to the ASIN, and the ASIN's popularity are factored into the likelihood you can materially edit that ASIN.
If you want your data from Zentail to flow through to and update an existing ASIN, it doesn't hurt to make sure that data is entered in Zentail. If it's a relatively incomplete ASIN with few sellers listed, and you have more clout (overall seller performance), there's a good chance Amazon's data system will accept some or even all of your updates.
For more information, please visit How do I control what data gets sent each channel?