Man, that's the real question right there. Why do we keep doing the math wrong on purpose, knowing we'll have leftover concrete or paint? Is it just the hassle of a second trip, or some deep worry that the store will be out next time? I even do it with soil for the garden.
Yeah, the part about measuring first to avoid extra mix... I see that everywhere. People buy way too much food for a small party, or get enough paint for three rooms when they're only doing one wall. It's like we're all scared of running out mid-job. I even do it at the hardware store, grabbing a whole box of screws when I only need five. That leftover bag of concrete hardening in the driveway is just the same feeling.
At my local hardware store, they charge double for loose screws compared to a box. So grabbing a whole box isn't ALWAYS about fear, it's about not getting ripped off. I've seen the same with paint where the gallon is barely more than a quart. We end up with leftovers because the math works out better upfront. That bag of concrete might harden, but at least I didn't pay EXTRA for a tiny amount. It's a cost thing as much as a worry thing.