PHREEQC Question on Charge Balance

Hello,

I've been a little perplexed on a few things involving charge balance in
PHREEQC. I am trying to model a simple mixing of 2 solutions, very similar
in chemistry, to yield a new pH.  The two initial solutions are charge
balanced with Cl. The mixed solution is charged balanced (based on the
output) by pH (or H+).  The charge balance for the original solutions are
listed to be very small (10-17), as is the charge balance for the mixed
solution (10-12).  However, when I manually calculate the charge balance
based on the concentrations given in the output for the mixed solution, I
get a value of 10-3 to 10-4???  Is this due to rounding of the displayed
results? (I've attached an example scenario).

I'm currently trying to do a similar calculation in Microsoft Excel - using
the charge imbalance of the mixed solution to define my H+ and pH but am not
obtaining realistic results.  How does PHREEQC use H+ to obtain charge
balance?

Thanks in advance for your assistance.

Attachment: TestScenario
Description: Binary data