Srscet allows the browser to choose which image to load and I believe in this case you probably had the developer tools opened in device mode when you first loaded so the 600x600 image was loaded for double density.
If you then go out of device mode and back to desktop the browser will look at the srcset and see that **you already have the 600x600 in cache ** so there is no point in going and fetching another image even if it is smaller as that would be a waste. I believe it is showing it as 300x300 natural because it assumes the 2x is double density and thus halves it.
Most of the above is just guesswork but I get the same results on Chrome when I follow the above. If I click refresh on desktop then the image shows 111x111 natural size for the 110x110.
Unlike the picture element the srcset leaves the decision making to the browser and of course browsers may vary but it makes it easier in the long run as the browser chooses the best image from those provided based on resolution and whatever may already be in cache etc.