WPT seems to always trigger lazyload?

I have a bunch of images that are set to lazy load on scroll. When I look at Chrome’s inspector / network traffic, I can see that those images aren’t loaded immediately, and only load when I start to scroll.

But when I run a WPT, the waterfall shows all the images loading. It seems like lazy loading is always triggered when WPT runs. Any ideas how or why that might happen?

Here’s a result link - notice the dozens of images being loaded. But if you visit the page and watch Chrome’s traffic, those images only load when you scroll.

https://www.webpagetest.org/result/170608_XF_39b8c2c702ef935dd5e5585161f39ed7/