When teaching a concept, it’s natural to suppose that the person you are working with isn’t able to follow each statement you make. This is a consequence of the fact that the material is new to them, which means it takes time to understand the material. I’m sure we can all think back to instances in our education where we only partially understood what was going on.
When you’ve understood a subject, there’s a tendency to take parts of the subject for granted. You make more assumptions about how things will work, and why one thing is used versus another. After all, you have spent so much time thinking about the subject that you know it inside and out. This familiarity implies potential sources of confusion don’t even cross your mind because you know how wrong they are. In other words, you start to figure that everyone else probably knows the same amount as you do, so certain things aren’t stated.
As someone who teaches, I struggle with striking the right balance between explaining a concept to a student and giving them the steps to solve a problem (the mechanics). At first glance, it might seem like these two ideas are the same, but a student experiencing difficulties will often need one without the other.