Rounddown formula discrepancy

wrt A4:
ROUNDDOWN(5.1,0) ---> 5
MATCH(5,$I$2:$R$2,-1^*) ---> 6
^*If match_type is -1, MATCH finds the smallest value that is greater than or equal to lookup_value (5 in this case). Lookup_array must be placed in descending order; no 5 therefore 6 is returned

Looks like an extension of this previous topic: https://www.excelforum.com/excel-for...wo-values.html

I think you need to spend a little more time understanding the -1 approximate match option for the MATCH() function that you are using. Using the 5.1 value as an example (follow along with the evaluate formula tool, if you want https://support.office.com/en-us/art...6-a70aa409b8a7 ):
1) 5.1 rounded down to the nearest integer is 5
2) 5 is between 6 and 4 in the descendingly sorted list. With the match type set to -1, Excel will see that 5 is between 6 and 4 and return the position of the number 6 (6th entry).
3) The INDEX() function then returns the 6th value in the list, which is 6.

For a round down type scenario like this, I would expect to have the list sorted in ascending order (like yesterday's list was) and use +1 as the match type argument. If I sort the lookup range in ascending order and switch the match type argument to 1, I get the values indicated in column C.