Take an everyday calculator and add up some numbers in this fashion:
111+333=444
Then Divide the result by two and you will get the value that is between the two chosen numbers which is:
222
It doesn't matter if you add 1+3=4/2=2 or even 111,111,111+333,333,333=444,444,444/2=222,222,222
How many numbers does not change the fact that the way in which we have numerical value aligned on a 1,2,3,4,5,6,7,8,9 Grid makes all sorts of interesting patterns show up..
For example you can also go cross ways.
777+333=1,110/2=555 which diagonally is the middle number