Why was UTC initially defined as being 10 seconds offset from TAI

Question

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?

Answer 1

Presumably because that was the difference between leap-second-free TAI (which originates from the 50s) and the 'leap second adjusted' UTC as introduced in 1972.