Hi Schopen, I'm gonna make an amateur's response to your question.
Disclaimer: I'm no expert, I'm just here to learn.
My interpretation is that "bullion price" is a more "generic" term than "spot price."
For example, over the course of one trading day, the "bullion price" can be "fixed" in New York, London, Hong Kong or anywhere else there is a "market" or an "exchange".
Those would all be "bullion prices" but they might change through the day as certain markets open and close, thus affecting the "spot price".
I hope that helps.
Good luck to you.