From http://derickrethans.nl/leap-seconds-and-what-to-do-with-them.html
UTC was defined (in the latest adjustment of its definition) as being 10 seconds different from TAI making 1972-01-01T00:00:00 UTC and 1972-01-01T00:00:10 TAI the same instant.
From https://en.wikipedia.org/wiki/International_Atomic_Time
TAI is exactly 36 seconds ahead of UTC. The 36 seconds results from the initial difference of 10 seconds at the start of 1972, plus 26 leap seconds in UTC since 1972.
Why the initial difference of 10 seconds?