Digital BW, The Print

I just started working with the UT2 inkset. I printed a 21 step wedge 
on Epson matte using Paul's curves in photoshop. The light through 
mid tone steps print with fairly linear gradations between steps. 
However, there is a sharp discontinuity moving into the darkest steps 
( 85 thru 100 %) where the black ink comes on strong. This is evident 
on on the CRT after the curves adjustment as well.
I'm not sure if I messed up or this is expected.
If this is normal,what is the reasoning behind this ?

Tom