Some times CO2 bubbles can float the hydrometer a bit. I either degas the sample or spin the hydrometer a few times to remove any bubbles.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk