ADC values, how/when does system convert raw to processed?

I am using the BBB's analog inputs to read some voltages. I am
finding that the 'raw' ADC readings read from the in_voltageN_raw
files don't seem to correlate exactly with the processed AINn values.

To test this out I wrote the following:-

    while true
        raw=`cat /sys/devices/ocp.3/44e0d000.tscadc/tiadc/iio:device0/in_voltage0_raw`
        prc=`cat /sys/devices/ocp.3/helper.15/AIN0`
        echo $raw $prc
        sleep 5

... and here are ten results I just got using the above code:-

1841 809
1840 809
1841 808
1842 807
1843 809
1842 809
1842 809
1841 809
1841 809
1840 809

Notice that the raw value moves between 1840 and 1843 but that the
processed result is 809 in both cases. Even more oddly the processed
result drops to 807 when the raw value is at 1842.

How quickly/frequently does the BBB convert the raw values? The
above might be explained by a bit of delay.