Observed TBT vs Main Thread Blocking Time by Script Origin

I’m trying to correlate the observed TBT time with the sum of blocking times reported on the Core Web Vitals results page.

For example the observed TBT is 3084 ms.
The blocking times by script origin are: 1185, 493, 38, 34, 0.
(sum is 1750 ms)

The sum is always smaller then the observed TBT.
What would be the reason for this discrepancy?