Two structural problems make process documentation harder than it sounds. First, you leave things out without realizing it. When you know something well enough, your brain stops flagging the intermediate steps. You write "make the sauce" when what you actually do involves checking the consistency, adjusting the heat three times and tasting at a specific point. The compression is invisible to you because it feels like one step.
Second, you can't see your own autopilot. Some of what you do happens so fast and so automatically that you don't register it as a decision. You glance at something before starting. You make a split-second call about which approach to take. You sense when something is "off" before you can explain why. You can't document what you can't observe yourself doing.
These two problems compound. You compress without knowing it AND you can't observe large portions of what you do. The result is SOPs that are approximately 40-60% complete while appearing 100% complete. The missing portion is precisely the part that makes the difference between expert execution and someone following instructions and failing in ways they can't explain.