Why are Progressive Web App Icons circular compared to their App counterparts

Working on a PWA (progressive web app) and coming up with the app icon for it. There aren't many others around since it's pretty new but it seems a lot of PWAs use a circular icon to differentiate from the native app.

Is there a particular reason they are choosing a circular icon compared to the many other options? Is this a standard or a guideline(coming from google it wouldn't be a surprise)?