Not all contacts imported from Hotmail to Google Contacts

Helping a friend move from Hotmail to Gmail. First thing on the order of business is importing the contacts. After exporting the contacts from outlook.live.com as contacts.csv and importing them on contacts.google.com I got an error message saying:

Imported 600 contacts out of 2377 found in file

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How come not all contacts were imported? How can I find out?