Syncronized Data Model relationship one-to-one vs one-to-many

I synced some Objects from SFDC to SFMC using Marketing Cloud connect.

Within Marketing, in the Data Designer, they appear as data tables and the tables have relationships between them, automatically created by the sync.

However, some relationships are marked as  one-to-one and some as one-to-many.

There are some one-to-one relationships that I believe are strange,

such as the one between Contact_Salesforce and Order_Salesforce. One contact could have multiple orders, could he not? Same between  Contact_Salesforce and  CampaignMember_Salesforce .

But for example, between Account_Salesforce and Order_Salesforce, thise one does get marked as one-to-many :s

Does anyone have an explanation for this?

Thanks,