Hi,

Please refer to the forum answer below;

https://www.odoo.com/sk_SK/forum/pomoc-1/how-is-the-vendor-on-time-rate-calculated-182460
and also please refer our blog

https://www.cybrosys.com/blog/how-to-manage-vendor-on-time-delivery-in-odoo-16
https://www.cybrosys.com/blog/how-to-manage-vendor-on-time-delivery-in-odoo-17

Hope it helps

The answer is, this Vendor often / always delivers more items than we have ordered. 

When we order 1 item and they delivery 2 items on time, we receive the 2 items and the on time rate shows 200%