Arduino Uno + Jaguar Speed Controller + CIM Motor (12V DC)
I searched for the library you're using and the arduino website mentions : 


  0 being full-speed in one direction, 180 being full speed in the other, and a value near 90 being no movement


From this we can take that the value 90 is not perfectly in the middle of the speed range therefore 10 points under 90 won't necessarily be equivalent to 10 points above 90.
My only advice is to try and find the equivalence point yourself by trail and error.
Even though 90 seems to cause no movement, try 89 or 91 see if they also have the result and so on.

Or simply try different values to get same speeds in different directions, it's ok if you use different values (away from 90) for the two different directions :P.

Gd Luck