Because either
a) The phone system isn't telling your PRI provider to divert the call and does it itself
b) The PRI provider will not accept a request to divert the call so the phone system must do it itself
c) Something in the call flow causes the phone system to do something on the call before the divert, eg play a RAD message.
After writing the above, I've re-read your original post. I am sceptical that there is a mechanism with PRI signalling to have the provider handle the divert after you've answered the call.