The data in the csv file is stored in octets and the GUI will be in Kilobytes, MegaBytes or GigaBytes.
The example below explains how we convert the data. If NFA is showing 11.33 MBytes for a conversation and the csv is showing 11876100. The columns in the csv show data in octets, to convert it to what is shown in GUI you need to divide it by 1024 to convert to Kbytes and by (1024*1024) to convert to MBytes.
Hence here 11876100 / (1024 * 1024) = 11.325 which when rounded becomes 11.33 Mbytes.