This happens occasionally when the software is updated after you've prepared the return but before you e-filed it. Obviously, some update affected the calculation of something on the return. Sometimes the software has a calculation flaw, sometimes new IRS (state) guidance comes out, and then there is the retroactive legislation our Congress had developed a habit of spewing.

My procedure is to compare the Form 8879 elements to the current software calculation prior to transmitting, particularly when there has been a long delay between the return prep and the return of the 8879.