Yes, ROI is Return on Investment.
The investment into a $5.50 tournament is $5.50.
So if you play a $1.10 Double or Nothing you get paid $2 (because they only double the non-rake part of your investment) which gives you an 81.82% ROI, not 100%.
It would only be a 100% ROI if you paid $1.10 to play a tournament and they paid you $2.20.