BQ76930: CELL voltage is quite accurate, but pack voltage has a big error.

Part Number:BQ76930

I use BQ76930 to protect 10 serial LI-battery. When  reading individual cell, the voltage error is only 2mV,each cell voltage is around 4.000V. So the pack voltage should be around 40.000V. But when reading the register-"BAT_LO/BAT_HO",it shows 40.500V. Then I use voltmeter,it shows 40.000V. The pack voltage that BQ76930 read has so big error, why? why not use a sum of individual cell voltage as pack voltage?

Thanks!