The quoted characteristic is quite typical, Vmo being an EAS consideration which gives the IAS variation.
It is likely, I suggest, that your constant Vmo source was a simplified airline direction solely to make the exercise easier for crew memory.
However, one expects that the barber's pole reflected the variation. My memory of the instrument is getting a tad scratchy but, as I recall, that was the case for our birds.
The actual AFM variation which you see on the gauge will be an amalgam of the various things which go into the melting pot for determining Vmo during the certification process .. EAS to CAS to IAS conversions, gust loadings, bird strike, etc. Figuring the actual story by reverse engineering might be an interesting exercise but probably a tad frustrating at the end of the day ..