service workers and TTFB on repeat view ?

First time playing with using service workers caching and trying to understand what is happening with WPT reported TTFB summary on repeated views - it’s very high at 1+ seconds but document loaded time is quick

see example at https://www.webpagetest.org/result/180721_29_a1063915a877d5b0878f1d5e24f1babe/

on repeat view i get

Load time = 0.072s
First byte = 1.159s vs 1st view = 0.154s
Start render = 0.100s
Speed index = 100
First interactive = >0.122s
Document complete = 0.072s
Fully loaded = 1.160s