Hi Whwoz,
I wondered about this question too, but I think as the genetics are still being worked out (especially the past decade) there is no clear definition as to the number of blooms that are properly called 'multiflora' in common parlance.
The genetics behind the 'clock' that times the action of various meristems in the inflorescence is plenty complex. Some of the genes involved do things like producing extra leaves/vegetative growth as well. There was a discussion of the environmental causes of this last summer, started by JRinPA, and quite a few of us had seen similar things which are changes in gene expression rather than genetic mutations.
I also have a breeding line that (couple years ago) started to produce 40-ish flowers in a cluster, when neither parent was a multiflora. It turns out that there are epistatic genes that can suppress the compound inflorescence genes - I found this work on " jointless" at the time, which could explain how this happened:
https://www.ncbi.nlm.nih.gov/pubmed/28528644/
There is also another gene discovered recently (besides "s" compound inflorescence) which is being called "bifurcated flower truss" bif, a mutation that increased flower number by X 3.3.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5920302/
This might be an even better explanation for what happened with my cross (by the numbers), if "bif" also has epistatic interactions with other genes which caused it to be 'unmasked' in the growout, as seems likely. Both the pink and the black lines have continued to express the large inflorescence so it is certainly genetic and appears to be 'fixed' as a recessive.
As KarenO pointed out, all of these large inflorescence variants have been referred to as 'multiflora' while genetic differences remained unknown... whether we will see bickering in future about whether they should all be called 'multiflora' remains to be seen.
