Interesting on the image you have posted above.
It looks like multiple small head would develop there but somehow they are flattened down.
I have something currently looking like that, which had quite the multiple heads like a cauliflower, after using a product from the pharmacy that contains Salicylic Acid and Lactic Acid, it now looks kind of how the image above looks.
Interestingly enough while that mix quickly reduced it's height to flat (2-3weeks with just a couple of application) it never made it disappear completely. It's true that I have not insisted in applying it for months.