partial Retirement

I am doing a partial retirement and getting a bit confused on the numbers the system is spitting in the 'Depreciation Amount' field in the 'Depreciation' tab of Financial Inquiry after doing a depreciation. The scenario goes like this:

Asset Cost : $1000

DPS: 1-Jan-2010

Depreciation Methid: 3 Yrs SL with Mid Month Proration

I did a depreciation till Oct-10 and the total dep amt was $263.91 which was correct.

Then I paritally retired cost of $500 and ran depreciation for Nov-10

The accumulated dep now is $145.84 which is right, however, for Nov-10 the system is calculating dep amt as $20.84 which I am not able to understand.