<human>: So, you can take any set of numbers, say (386, 7832, 21, 846, 1062) and then you add them up: (10,147). 
Then you add up the digits: (1+0+1+4+7=13), and keep adding up digits until you get a single digit: (1+3=4).
But we can also just add up the original digits, like this:
(3+8+6+7+8+3+2+2+1+8+4+6+1+0+6+2=67) and keep adding digits: (6+7=13), (1+3=4) Again.
And you can even add up the numbers in groups and then digitise: (386+7832=8218) and (21+846+1062=1929), so now we have (8218, 1929) and again we add up digits: (8+2+1+8+1+9+2+9=40), (4+0=4) Again.

Is there any explanation as to why this happens?

Can you summarize the given text in a few sentences?
<bot>: root digitising a set of numbers will bring the same final digit as adding up those numbers and then root digitising or adding up groups of those numbers and then digitising.