Super Transformation will cost not only 1 magic rock if keep pushing hotkey when casting other skill

Since my skill level of Super Transformation is only 1, so my skill duration and skill cold down are both 60 sec.
Therefore, I always recast Super Transformation by keep pushing hotkey to Super Transformation again when the duration is almost over or the skill animation after I leave Super Transformation is over.

However, I found that Super Transformation will cost not only 1 magic rock if I recast Super Transformation by keep pushing hotkey.

For example, in the following video, the Super Transformation cost 9 magic rocks if I recast Super Transformation by keep pushing hotkey when the char casts Dragon Strike.