I don't know the answer to that without looking, but I can tell you how to figure it out.
These guys didn't use numbers themselves. The numbers were assigned retroactively, typically in the 19th century, by genealogists who wanted to eliminate confusion. Unfortunately, they sometimes used different rules and came up with different systems.
So, you can usually figure out the different systems by looking at the whole family tree.
For families with an ancestor in Normandy, the numbering sometimes starts before the family came to England. Other times, the numbering starts with the first ancestor in England, even though there was an earlier generation with the same name in Normandy.
For families where the succession went from grandfather to father to son all with the same name, the numbering usually follows just the heir. Sometimes, however, the numbering includes younger sons who didn't inherit the estates.
If the surname changed because of different estates, sometimes the numbering starts with the earliest male ancestor even though he had a different surname. Sometimes it starts with the first man to have the surname.
When an man inherited, left no children, and was succeeded by his nephew, the numbering generally is supposed to include him, but sometimes he got missed in the counting.
Sometimes the numbering is extended to include cousins. Sometimes even distant cousins.
Sometimes historians figured out later that two men they thought were the same were actually different, so the numbering had to be re-done to include an extra person.
So, to find out why Saier is sometimes III and sometimes IV, you'd want to look at the pedigree view for an earlier Saier who is either not in the direct line or who lived before Saier I. Somewhere in there you should see four Saiers, and be able to see why one of them might not have been given a number.