Multi-Period accouting giving strange results

EBusiness 12.1.3

We set up multi-period accounting according to directions in the white paper for use with prepaid invoices. It is working with the exception of the fact that it is calculating strange amounts.  As an example, I created an invoice for 12,000.00 and asked to have it amortize over 6 months.  I would expect it to recognize 2,000 each month but that is not happening.  For the first five months it recognizes 1,989.50 and in the final month it is recognizing 2,052.50.  Has anyone seen this before?  Any idea what might be causing it and/or how to correct it?  Thanks.