This is a bit of a confusing situation, so let me try to explain.
srcset attribute contains information on what images are available for use in this
<img> element - it defines their filenames, and what their real width is (with the “w” unit).
sizes attribute contains hints on what size slot the image will be expected to fill at a certain media condition. So here we are saying “when the viewport width is 480px or less, the expected slot size is 120px wide, otherwise the slot is more like 400px wide.” there is no exact sizing specified here - these are hints designed to allow the browser to make a sensible choice of what image file to load, based on the videwport size (or other media conditions, if specified).
So the effect this has is that if the viewport of your browser is 480px or less, the 120px wide image is loaded, but if not, the 400px image is used.
You can test it out; load this example: https://mdn.github.io/learning-area/html/multimedia-and-embedding/mdn-splash-page-finished/
Now open the browser dev tools, go to the Network tab, reload if necessary, then look at which dinosaur head image is downloaded. It should be the 400px one.
Now reduce your viewport width to really narrow, then do the same. You should now see that the 120px image is being loaded.