Most people don't realize that because it isn't really true...
I mean, each PSU has its own power efficiency curve. Perhaps you have one particular model that's an outlier but - here, let's look at a few:
SuperNOVA 850 T2 appears to achieve peak efficiency around 400W, which is 47% load
SuperNOVA 850 G3 peaks around 400W as well, which is another 47%
SuperNOVA 650 P2 looks to peak around 250W, which is 38%
CX650M is peaking around 300W, which is 46%
Now to be fair, I'm guessing the peak value from a visual inspection of the curves. It's possible they mathematical peaks are a little lower, which would bring some of the values closer to your 20-30% range, but power efficiency falls off hard at light loads as opposed to slowly at high loads, so personally I'd rather err on the high side.
For bonus points, here's a post talking about oversized PSUs being most efficient as a myth. It acknowledges there is performance variance based on the load, but suggests the ideal load for PSUs is usually more in the 40-80% range.
Myth #1 is that using an oversized power supply will consume significantly more power at idle than a smaller unit since PSUs are less efficient at low loads and high loads, and are at their best between 40% load and 80% load. This is true, a power supply is less efficient at low load and there is a "curve" if you plot a PSU's efficiency over a variety of loads and zoom in real close by truncating your Y axis on the graph; however, if you look at the graph from 0 to 100% efficiency it is a pretty flat line between 10% and 100% load for most units with 80 plus basic certification or better, and the graph doesn't plummet as it approaches 0% load as many people draw it