Inspection of onLoad and tffb appears to show a few anomalies w.r.t 20-80 rules.
Enclosing three random cases, where the reason appears to be redirects? In the cases, the ttfb >> onLoad. All the cases have 1-2 redirects before the actual HTML is served.
Date: 2018_03_15_desktop
SELECT
pageid, time, status, type, format
FROM
httparchive.summary_requests.2018_03_15_desktop
WHERE
pageid IN (87277828, 87291432, 87267343)
ORDER BY pageid
87277828 onLoad=340/ttfb=478
87267343 onLoad=352/ttfb=1939
87291432 onLoad=212/ttfb=570
The ttfb appears to include the time taken by all the HTML requests (including redirects), but the onLoad appears to be for the last HTML.
In such a case calculating ttfb/onLoad for the backend/total time ratio may include a bit of error. Using the ttfb of the HTML with 200 status (by looking at the requests summary table) may be more accurate.