If you set it for 8 rings, you're going to hear 9 or 10 before voice mail picks up. For fun, set it for 3 rings in #321 and try again.
I'm guessing that the reason it covers when transferred by the auto attendant, is you didn't set the Transfer Return Extension for each voice mail port to Ext. 10. If that were the case, transfers to extensions without VMS coverage would return to Ext. 10 after the Transfer Return Rings expired. With the Transfer Return Extension setting for each voice mail port set to itself, the call would instead return to the voice mail on no answer. Older voice mails could not interpert the return, and would send the call to the Operator extension, but Partner Messaging will find a matching mailbox if available.
You will want to set your Transfer Return Rings LARGER than your VMS Cover Rings.