How does BGP work in this case?

Question

Computer networks

Colleagues, welcome!
Please help with the following question: the company's network is connected to two providers, one provider acts as the main one, the second as a backup.
From network engineers, we require that all traffic go through the main channel by default, and in case of failure / unavailability - through the backup.
But engineers report that 5-10% of the traffic will still come through the backup provider. Explained as follows: this is how BGP works.
When announcing company networks through providers, priorities are set, but when exchanging technical records with other providers, and those with others, there is a "spread", as providers can interpret priorities in their own way.
I'm not a network engineer and I can't argue with them with arguments :)
Colleagues are really right and what I ask them to do is impossible?
Thank you!

Answer 1

Yes. The engineers are right. this is how BGP works.

Answer 2

In part, they are right, but if you make a long AS Prepend, then I think that these 5-10% will decrease to 0.
PS I have not seen providers that cut off AS prepend, although everything can be.