That's because it's also counting farmer, steppe and even WHG that the Guanche sample has:
Target: Portuguese
Distance: 3.0056% / 0.03005567
53.8 Anatolia_Barcin_N
29.8 Yamnaya_RUS_Samara
10.8 WHG
5.6 MAR_Taforalt
Target: Canary_Islands_Guanche
Distance: 1.8774% / 0.01877371
34.8 MAR_Taforalt
27.6 Anatolia_Barcin_N
16.0 Anatolia_Tepecik_Ciftlik_N
7.8 Yamnaya_RUS_Samara
5.2 Levant_PPNB
4.4 Yoruba
2.2 WHG
2.0 Dinka
Either way, if i run this calculator with the portuguese samples avaliable on G25, the average for NA is 8.6% and one individual scores 15% while another scores 1.6%. So if someone scores 18% it means he scores way above average.
(the image doesn't have all the samples, but at the bottom i could see the average for Guanche was 8.6%)
https://i.imgur.com/fYthM8l.png