Grasp vs understanding

Hello,

How would you differentiate between: "Sharpening edged instruments is a skill and it can be learned, but you need to have a very good grasp/understanding of the concept of angles..."?

To me, "understanding" is deeper (also stronger than grasp) and suggests that the person has more knowledge than someone who has a (good/decent) grasp of something. I may be wrong, but grasping something involves a certain degree of difficulty, doesn't it? Some dictionaries list them as synonyms, but for some reason I am reluctant to agree with that.

Thank you.