I tried experimenting the unit tester custom map.

A fully upgraded ultralisk, which would have a total 7 armor vs a zergling with a basic attack of 5.

By right, the zergling would not do any damage to the ultralisk coz the ultra has 7 armor compare to the zergling who attacks with only 5.
But when I do a testing, the ultra received about 1 damage for each attack from the ling.

This also goes to the larvae which has armor i think about 10.

Anyone can explain?