system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does my video still lag when the buffer bar says it's buffered up to 5 minutes ahead?

If it's buffered already, why won't it just play smoothly?
gpt
People who don't understand how computers work are way over complicating things here. The real reason is that the video isn't actually ready to play, but the buffered bar is shown to generate a placebo effect for humans. The people who programmed the player make it overstate how much of the video is ready to play because it makes it seem as if the player is working better then it actually is. 

There's actually a good reason not to buffer far ahead, particularly for internet video. If they buffer the video far ahead, they have to transfer all of that data to you. This costs money for them and for you. People don't always watch the whole video, so if they transfer all of that extra data that doesn't get viewed, they've wasted money. That's why they program the player to lie a little bit. It makes it seem as if the player is doing more than it actually is, while cutting costs for everyone. If the whole video plays smoothly, everyone is happy that it worked so well and the video seemed it was ready ahead of time for them. If it doesn't play smoothly... well, you get this question.

TL;DR: The video isn't actually ready to play, the player lied. The video is the Christmas gift, the player is Santa Claus, and your parents (programmers) told you about Santa.