Why do I always think I'll be productive on my days off and then do almost nothing?

Every friday I tell myself that this weekend is finally the one where I'm going to get everything done. I'll make a mental list of things like cleaning, organizing, going to the gym, running errands and maybe even starting a project I've been putting off.

Then the day actually arrives. I was playing on my phone for what I thought was 15 minutes, and somehow it turned into over an hour, unnoticed. I've had lunch, watched a couple of videos, maybe gone out for coffee and suddenly it's late afternoon. I haven't done nothing but I've definitely done a lot less than I imagined I would lol

Is there a reason our brains seem to overestimate how productive we'll be when we finally have free time? I look forward to my days off all week but they almost never play out the way I picture them in my head.