Why are impairment results inconsistent?

My users ran impairment via the impairment worksheet to bring all the asset values to 0 within a particular department.  We have done this before, and it ends up creating a contra asset with the depreciation expense exactly matching the depreciation expense for the existing asset for each month.  This time, for some profiles it did this, but for one profiles it did not.  For that profile, we do have the depreciaition expense for the contra asset, but it does not match the depreciation expense of the existing asset.

What could have caused this?

And more importantly, what can we do to correct this?