I have Maxxis ikon tires on my 29er that are 2.35's front and rear, but wanted to go with a bit larger front, so I got a Ardent by Maxxis 2.40. Why would the new larger tire 2.40 measure 4mm less in width?
Maxxis tires have a long history of being undersized, but some of their tires now are true to size. If you want volume, look at Schwalbe HD or MM 2.35".