You had an hour. It was split into a twenty-minute block, a ten-minute gap, a half hour. By the end of the day the hour is gone and you never started anything. The time was there. It just didn't feel usable.
The experience is real and it has a name.
What's Happening
Researchers call this the attention residue problem. Sophie Leroy at the University of Washington showed that when people switch between tasks, part of their attention stays stuck on the previous task, and the smaller the block, the more the residue dominates what's left (Leroy, 2009, "Why Is It So Hard to Do My Work?"). A ten-minute window isn't ten minutes of focus. It's two or three minutes of focus wrapped in residue from what just ended and anticipation of what's next.
Your brain knows this instinctively. It looks at a twenty-minute gap and correctly decides it's not enough runway to start anything meaningful, so it scrolls instead. The problem isn't procrastination. The time really isn't usable for the kind of work you're trying to fit in it.