Timing of posts marked unread

When I read a thread or topic (whatever) to the end and then leave it, it sometimes marks the topic as partially unread. Looks like it’s assuming it takes a certain amount of time to read them and if you don’t spend that much time there, it assumes you didn’t read all of it. I never thought of myself as a super fast reader, but apparently I’m faster than whoever came up with this thinks is typical. I suspect I’m not alone in this.