Poor GP post is going gray, but I kinda agree. 13.3 should probably make the <13 cut, just as 14.9 should make the >15 cut. The slider is limited to whole numbers, so it might make sense to round the display's actual diagonal to the nearest integer before applying the user's criteria.
Your definition of "up to 13", includes the higher number 13.3? I hope you aren't a programmer.