Eloqua contact mapping to Custom Data Record

Hello! :-)

I see the example of the custom data record mapped to the incorrect contact.
Is there any possible way to see the reason behind this unexpected behavior?

Around 100k contacts were uploaded to our database - new contacts and few edited.

After the contact upload, we uploaded the Custom Object with the same data (used the same excel file) and one of the contacts got mapped to the incorrect contact. The odd thing is that the contact record got mapped to the incorrect contact - but the contacts first and last names are identifical.

I am wondering when eloqua does the mapping part in the system custom data record updates does it take into account all the email address or for example only the first 15 characters and hence we see a discrepancy? The other custom data records are mapped correctly.