Is this discrepancy above due to a calligraphical mistake or lazyness, namely, leaving off that one tick, made a long time ago?
TL;DR: I don't think so, because (1)
尸 used as the pictographic radical with the meaning
屋 was explicitly mentioned in 说文解字 long before, (2) the original meanings of
户 used as radicals are not exactly the same (
尸 used as the pictograph in
屋 was explicitly mentioned in 说文解字:
尸 looks like house.)
This article summarized three usages of
尸 when used as radical.
(italics is translation from me)
A.从原意而来，代表人的躯体。(Derived from the original meaning, represents body of human.)
B. 从履字来的，表示各种鞋子的字。履不是人体的部位，但是是服装，所以和尸的原意仍然有关联。(Derived from 履, used to represent various kinds of shoes. 履 is not the part of body, but the clothing, it's still relevant to the original meaning of 尸.)
C.第三类的尸本意不是躯体，而是指房屋。其形状像一面墙壁和屋顶。(The meaning of the 3rd kind of 尸 is not body, but the house. Its shape looks like a wall and the roof.)
户 means single leaf door originally, note it's not exactly the same but relevant to
Here's a picture from the book "字源谈趣" explaining the difference between
户, especially their ancient form (left-most one).
Now we could understand the original meaning of
正室左右的住室 (wing room/side room (with single door))
Additional information about
居 seems to be the different case with
居 is the original character of
squat. Then it was used for other meanings,
踞 was created for the using of the meaning
squat. So the usage of
居 should be the 1st case shown as before.