arithmetic – Not understanding simple mathematics

I’m 21 and a university student, studying a STEM major. Here are just a few examples of basic arithmetic that I don’t understand:

  1. Sometimes it takes me much longer than normal to do a division problem (For example, if you have 10 apples and someone eats them at 2 apples per minutes, how many minutes does it take for the apples to go away?).

  2. I didn’t understand why multiplication was commutative until I was 20 (why a x b = b x a). I finally understood that it’s true if you visualize the multiplication as counting the squares in a rectangular grid.

  3. I didn’t understand that the remainder in the division x/y can be interpreted as how many things are left over after you distribute x things evenly to y people (I only interpreted it as how many things are left over if you try to make as many y-sized groups as possible from x things).

However, in more advanced math classes, I do well (sometimes understand things conceptually well, and definitely do well with regards to grades). Anyone have insight on why I might struggle with these simple things?