Yes Icelandics do, as do Swedish Flower Hens for instance which are both Landrace breeds. They have been bred for body type, size, foraging abilities, cold/heat tolerance, and laying capabilities. But color and pattern has not been selectively bred for. Similar to the way some American breeds such as Plymouth Rocks and Wyandottes but they have also been bred for color and pattern as well. Thus they could be standardized by the APA because the colors breed true from generation to generation.
The same thing is going here with the OP's chickens. They have very likely developed for their various qualities over many generations. But breeding for a specific pattern or color has not been.
I completely agree with you, the do seem to have the same overall shape and conformation.