I just used another calculator using different measurements (waist, hip, chest as opposed to waist, hip, forearm, wrist) and it says 17.6%. That seems more believable for me since I have no visible abs.
That test is far from accurate, so I wouldn’t pay much attention to the results. Even if they are correct at 13.5%, it is quite possible that you still don’t have visible abs. Strongly defined abs don’t show up till under 10% typically, with some definition showing up probably at 13% or less. Everyone is different though, it depends on how muscular your abs are, body fat, and just each person’s genetics.