TTI metric wrong? (main thread looks busy)

[zzo]

Here is another example: https://www.webpagetest.org/result/19010...view_step1
How can first interactive be at 32s?? Looking at the 'Page is Interactive' chart there is a ton of activity on the main thread between 30-45 seconds - there is no 5 second period of inactivity there - especially around 30 seconds.
How can this be??? thanks!!