Why does GTmetrix give me the “serve images that are appropriately sized” error?

I’ve run a GTmetrix test on my website.

I get one issue in its audit report at https://gtmetrix.com/reports/www.datanumen.com/3FtNGzq8/

audit report

I check the image with Chrome DevTools, as below:

chrome devtools

Since the test is for Desktop, the image size is 649 x 508 and it is served as 649 x 508.

Also I found that there is a srcset tag in the code for the different resolutions, which is also recommended in https://gtmetrix.com/blog/how-to-properly-size-images/. So I cannot find the problem with the image, why does GTmetrix report it as an issue?