Well NASA has history with making $300 million mistakes with units of mesure:

http://en.wikipedia.org/wiki/Mars_Climate_Orbiter#Cause_of_f...

  The primary cause of this discrepancy was engineering error. 
  Specifically, the flight system software on the Mars Climate 
  Orbiter was written to take thrust instructions using the 
  metric unit newtons (N), while the software on the ground that 
  generated those instructions used the Imperial measure 
  pound-force (lbf). This error has since been known as the 
  "metric mixup" and has been carefully avoided in all missions 
  since by NASA.

Are we even sure of the type of ton used in the explosion measurement?