"But it is a bipolar converter, so it ranges from -20,000 to +20,000"

I dont think it is that way. For each measurement the ADC uses 40,000 clock cycles ( counts ) to carry out the Auto zero, Integrate(10,000 counts) and de-integrate (20,000 counts). And in the process of de-integration, if the comparator is unable to "zero" the refernce capacitor even after 20,000 counts have elapsed, then the over range bit is set.

I know its bipolar, but each measuremnt is limited to a maximum of 20,000 counts. The rest of the 20,000 counts are bascially for housekeeping..

Raghu