You're right - it's not. Your system may be writing to the USB longer than you think it will be. That's because your computer splits its tasks up into segments, and it does a segment here, a segment there, so that it looks like you're getting good response time. In reality, it may take time to save a file on a USB, even if you think it's had plenty of time to do it.
Look for a small icon on the tool bar at the bottom of the screen. It's usually a small green square. Hover over it with your mouse - it will say something about removing a USB drive safely, or some such. Click on it, and you'll get a small box with a similar message. Click on that, and in a second or 2, you'll get a message that it's safe to remove it.