Don't get all wrapped up in charts, graphs and numbers. They can be deceiving, unless you look up the actual test parameters. It's not as simple as drawing a line across the chart. It's a lot more complicated than that.
So Just think about it for a minute. A multi-weight
oil is designed for a specific purpose. The "5W" part is thin, so it flows easily when it's cold. The "40" part is a bit thicker, so it supports the bearings, rollers, and such when the engine is hot. It's not logical for it to be the other way around. You wouldn't want something that's molasses when cold, and baby
oil when hot.
Would you want to be driving around all the time with a 10W
oil? Certainly not. If that were good for engines, we'd all run a straight 5W oil and be done with it.