updatePolyfill is called immediately after init for all images

After the first call to lazySizes.init(), the following chain runs right away: init → loader._ → checkElements → unveilElement → (rAFIt) lazyUnveil → updatePolyfill

updatePolyfill is being invoked immediately for all images on the page. Please help me understand why it triggers right after initialization (even before any user interaction) and whether this is expected behavior.