Try moving your chair - you may be sitting on a nodal point in the room.
However, this is a classic case of trusting your ears rather than your eyes. If the bass sounds clean and tight, then why worry about what the graph looks like...
The kind of music you listen to in relevant too. A lot of music doesn't have that much energy around the 40hz region - which is the low E on a bass guitar.
I couldn't find any measurements on the Tyler speakers website, but I wouldn't be surprised to find that 40hz is the port tuning frequency. You could always block the ports with foam and remeasure to see if this is the case.