Visual progress reducing over time - what does it mean?

I have been running some tests and I see the Visual progress appears to reduce and then increase over time. I can understand when this chart holds at a level and then increases towards 100%. However, I am not completely sure on what it means when it reduces from current value.

Here’s the comparison:

http://www.webpagetest.org/video/compare.php?tests=161104_KY_549d66fd0cab86db8386be14abe241ee%2C161104_DG_8ea5678984f5a3499df85e6a5166b127&thumbSize=200&ival=100&end=visual

I know that the base page in this case contains video and progressive JPEGs. Is this the cause of such a variation?

The visual progress compares to the end state and it is being thrown off because there is a large full-page interstitial that covers the entire page.

Yes, the page does build out with lazy-loaded content. This makes sense.

Thank you, Pat!