By default you are correct, but I checked with a coder to be sure before making a post. Our coders made a set of code that will round stuff off, so 18.5 becomes 19 and 18.4 would become 18. I was pretty sure it did but wasn't 100% sure what the break off was, so had to check with a coder.
But NPC's don't have their appearance tracked the same as Sveta, they use a much more simple system. But my point was most of the girls that have set models,those models are in the ideal BMI range with 3 being underweight. The three underweight are hotcat 6, 7 and 8. The 6 and 7 is totally something you the player could achieve with the same BMI. The 8 is the only one that is borderline, so all and all I think we did a fairly good job with the girls.
Also the numbers you listed assume their other stats are the same as BMI is only one thing that effects it, skin condition, physical stats etc all impact. So in your example if Anushka and Sonia had the exact same physical stats, teeth, skin condition, etc and we didn't have the rounding system which we do so makes it moot, but for example say Sonia was 18 BMI then yes your example would be correct