Outlook 2013 Export/Import problem

Hi

I am exporting emails from an old outlook 2013 Exchange account using the Export wizard.

I am also choosing that email account as the folder and "Include Subfolders" checkbox selected.

The file gets properly created as a pst.

However, on importing the pst into Microsoft 365 the original folder contained about 1500 emails while the imported one only has about 600.  

Kindly advise.