The floor command some times rounds a whole number down to the previous whole number. For example, I have observed the following behaviour.
2.6 * 7 - 0.2 = 18 floor 18 = 18 floor (2.6 * 7 - 0.2) = 17
This was experience in Arma 3 version: 1.64.138732
I found another example
2.6 * 12 - 0.2 = 31 floor 31 = 31 floor (2.6 * 12 - 0.2) = 30
I'm pretty sure this is normal floating-point imprecision which can't really be fixed.
2.6 * 7 - 0.2 isn't exactly 18, but something a little less.
Try putting this to the console's watch field and see if it returns false:
(2.6 * 7 - 0.2) isEqualTo 18