Update Import creating new listing

Hi,
I imported listings which had some data errors, so then i trashed+empty trashed those, and then imported again, and the same number of listings got added again. I also ran term count tool after this.

Then i had to correct some field values in some of the listings so i took an export, edited that (deleting some columns like post date,modified date) and then imported the selected listings again- Update if ID exists and with ID field populated

However, i found that instead of updating the listings, new listings were created again..with lower IDs, possibly from the ID ‘gap’ that existed from the listings i had trashed earlier.

Why would be creating new listings which the import file has ID and “Update if ID exists” is selected? Would it have anything to do with the listing trashed earlier and the numbering sequence gap?

Or do i not need to delete post_date adn modified_date?

Please let me know how to fix this?

thanks,
Vivek