What is the reasoning for step 4 of the Zebra puzzle, "The green house is immediately to the right of the ivory house"? Because you (or your program) automatically filled house nr. 4 as Ivory and house nr. 5 as Green. Based on the current information, Ivory could've been 3 and Green could've been 4.

The same for step 9, the Spaniard and the Dog are filled in house 4, but could also be put in house 5, based on available information.