Just giving my input here.
There are 2 ways that could cause this issue.
Imagine this diagram as your Iron Farm
++++++++++
+--------------+
+--------------+
+------X------+
+--------------+
+--------------+
++++++++++
X Represents your village center which is the center of your Iron Farm.
A door within a 60 block radius of this farm could cause something like this;
++++++++++
+--------------+
+--------------+
+--------------+
+--------------+
+--------------+ X
++++++++++
Which means your iron golems could spawn outside of your farm and in the slabs like you were saying.
The other issue could be something like this;
X still represents your village center
S would represent the possible spawning place of the golems.
++++++++++
+--S-------S--+
+--------------+
+------X------+
+---S---------+
+-----------S-+
++++++++++
Now if you restrict the space of the spawning spaces, you could have this issue here;
+++++
+-----+-S
+--X-+
+-----+-S
+++++
Hope this helps!
