I’d be very careful with “published” GI values if the source is not disclosed. As Norm says many foods have not been tested. And even those we have values for are often tested only by about 10 people. In general, this leads to rather vague numbers like 12 +/- 4 for a particular Greek Yoghurt published in Atkinson FS, Foster-Powell K, Brand-Miller JC: International Tables of Glycemic Index and Glycemic Load Values: 2008. Diab Care 2008; 31(12).
The UoS driven site http://www.glycemicindex.com/ shows only the value “12” at first glance. When you click the name of the food in the listing you get to the more detailed info, for this Greek Yoghurt it is http://www.glycemicindex.com/foodSearch.php?num=1275&ak=detail where the SEM value is the error margin +/- 4.
For the FTD I recommend the conservative approach: If in doubt use the lower numbers. Fortunately, e.g. with yoghurt the GI simply doesn’t matter much because of the low amount of carbs.