Seems this function is reading the display value property rather than actual value of the cell.
If cell has 8.456784 with a number format of 1 decimal, ExportDataTableAsString is reading 8.4 instead of full value. Can someone look into this and confirm. Thanks.
Thanks for providing us some details.
I did test your scenario/ case a bit using a simple sample file, it works fine and as expected. Could you provide your template file and paste your sample code to show the issue, we will check it soon.