Perhaps it simply comes down the level of interest and dedication of the designers involved?
When it comes to unique, monstroid figures like Mantenna and Modulok, they're interested so they care more about the final results. And that care is reflected in the final, production action figures.
But, for Double Trouble (Mischief) and others like Frosta, they lack that interest and therefore simply do the bare minimum to translate the Four Horsemen's proto-type into the final, production action figure? It's a shame that it has to be that way. The decline in quality and visual appeal between the 4H's proto-type of Double Trouble and the production figure is obvious, yet it somehow escaped the attention of Mattel's in-house designers or they simply didn't care enough to correct it or to pursue an alternative (e.g. swappable heads with a shared helmet/hair).