Why API response Data structure of a Custom field in Zoho Books or finance applications is different from Zoho CRM's Custom field

Why API response Data structure of a Custom field in Zoho Books or finance applications is different from Zoho CRM's Custom field

Hi Team,

As per the topic, I would like to know why the customs fields in Zoho CRM is acting or treated like other system fields in Zoho CRM.
But in Zoho Finance tools, the custom field is shown under the key "custom_fields" as a list.

I would like know about the reason for this. 
Because of this structure, I could use this fields for search api's etc.,

Could anyone help me understand this.