This may take me a while to wrap my head around. What exactly is the OFFSET returning to the expression? And why the -1?
Offset takes a cell reference (e.g., $K$3) and moves you up/down a number of rows and/or left/right a number of columns you choose. So you lookup the ATT in your data table, find the index, and, starting at the first ATT level cell (Str), move down INDEX number of rows - 1 (so STR index 1 - 1 means don't move down at all).
You could start at the row above the Str Value, but if you later decide to insert a new row for some reason between the header and Str, it would mess up. You could also adjust the indexes to be 0-7, which would allow dropping the -1.
Edit: Actually, you can drop the step looking up the index by doing a VLOOKUP for exact match.
So this: LOOKUP(LOOKUP(LOOKUP(M28,'Data Tables'!E$15:E$22,'Data Tables'!F$15:F$22),D$3:D$10,K$3:K$10),'Data Tables'!M$15:M$25,'Data Tables'!N$15:N$25)
Becomes this: LOOKUP(VLOOKUP(M28,$E$3:$L$10,7,FALSE),'Data Tables'!M$15:M$25,'Data Tables'!N$15:N$25)
Making the following the new somewhat shorter expression:
=IF(LEFT(M28,3)=RIGHT(M28,3),LOOKUP(VLOOKUP(M28,$E$3:$L$10,7,FALSE),'Data Tables'!M$15:M$25,'Data Tables'!N$15:N$25),LOOKUP(VLOOKUP(LEFT(M28,3),$E$3:$L$10,7,FALSE),'Data Tables'!M$15:M$25,'Data Tables'!N$15:N$25)+LOOKUP(VLOOKUP(RIGHT(M28,3),$E$3:$L$10,7,FALSE),'Data Tables'!M$15:M$25,'Data Tables'!N$15:N$25))
