I'm pretty sure you are right in your identification of your plant as M. heyderi ssp heyderi. To be honest, I find separating some of the subspecies quite difficult, but the higher radial spine count does give support to ssp. heyderi.
It is described as flat-globose to globose, and really it depends on how much water it has. In winter, I'd expect it to flatten and draw down closer to the soil, and then when it has the first water, it will plump up. I think that is what you are probably seeing.