Tire Rack shows 5.51" of backspacing on the 18x9 XD Monsters with 18mm offset, I thought it was 5.71" Just curious as to why it shows that

 

No. That is correct on that wheel. Actually it come 3 different ways.
18x9 with 35mm offset and 6.37" backspacing (fits a stock truck with no mods)
18x9 with -12mm offset and 4.52" backspacing (Will allow wheels to stick out)
18x9 with 18mm offset and 5.51" backspacing (Will allow wheels to stick out)