Why is changing the currency of a contact not allowed?

Why is changing the currency of a contact not allowed?

My contacts in ZOHO Invoice come from ZOHO CRM automatically.

Some of the accounts in CRM have the currency set to EUR, while others to HUF.

When trying to edit the contact in ZOHO Invoice, the currency field is disabled for one account, but enabled for another. What is the rule, why is the currency disabled in some cases?