The person explaining a concept can forget the years that made their shortcut possible.

Expertise erases its own staircase. The shortcut you use now was built on years of examples, mistakes, and slow work, and once it is automatic you cannot see the steps any more, so you offer the compressed version to someone standing at the bottom. The explanation is accurate and unusable.

When explaining something you know well, start from what the other person can already do rather than from your summary. Ask them to explain it back and listen for where it breaks. That is faster than any amount of clearer restating.