INDEX Issue

It looks like the problem is really within the MATCH() part of the function, and appears to be specific to the "between" tests in the search array building part of the formula. Specifically, (using row 22 as an example) you are asking Excel to find where a text string "95+34.92" is between the text string "93+00" and "117+00" Excel will perform the test using text sort order, but it appears you want to perform the test in "numeric" sort order -- seeing these text strings as a composite of two numbers. In text sort order, both "93+00" and "117+00" are "smaller" than "95+..."

You've identified the problem as you wrap around at 100, but I expect you will have the same problem in the first "0+00" to "12+00" group as well, since "8+yy.xx" will not return 1000 and 5, but will return 1003 and 8, because "8+yy.xx" will be between "66+00" and "93+00". Your sample file did not include any tests from that group, so you are missing that error.

You've indicated that you are limited in the changes you can make, so I'm not sure what changes would be allowed. My first thought would be to change everything to "xxx+yy.zz" text strings, with leading zeroes where needed. "095+..." will be between "093+00" and "117+00". The other solution that comes immediately to mind is to convert these "number-like" text strings to real numbers in some way. Maybe replace "x+yy.zz" with a number x.yyzz. "95+34.92" becomes 95.3492 which is naturally between 93.0000 and 117.0000. I cannot know if those kinds of changes would be allowed.

One way or another, you've got to either make your text sort order "between" mean what you want it to mean, or you've got to transform the data to numbers so that "between" will mean the correct thing as numbers, or you've got to expand your search algorithm so it knows to search 1 digit x+ differently from 2 digit xx+ differently from 3 digit xxx+.

Something has to change, but I'm not sure what you are allowed to change.

Removed as MrShorty has explained all!