I am having the same issue with the CycleOps H2 and the Assioma. When I use higher gears, the H2 reports higher power.
It seems quite common, and due to the flywheel inertia.
Answers here: https://m.youtube.com/watch?feature=youtu.be&v=_Pji1uAu_Us
And here: Big vs Small Chainring - Same Power (ERG Mode Gearing)
My original post here: