Intensity Minutes calculation is wrong

On a 8km walk I registered 28:15 in zone 5, 29:18 in zone 4 , 11:29 in zone 3, 4:02 in zone 2 & 0:38 in zone 1 -- how can this possibly translate to 71 mins moderate + 4 min vigorous ?

Also if this activity is statted as a Walk, why does it not appear under Walks, but only under Activities ?