What is trilogy but 9?

What is the significance of the phrase "trilogy but 9"? Could it be referring to a series of three works, such as books or films, but with one element missing, hence the "but 9"? Or is it a cryptic reference to a mathematical or numerical concept, where "trilogy" might suggest three elements, and "but 9" implies some form of subtraction or exclusion? It's intriguing, as it seems to suggest an incomplete or altered version of a traditional trilogy. Could you elaborate on the context or meaning behind this expression?