Comparison of two keys in reduce method?

[arvindkyutech]

I am visiting the codes in 'dd.py' file of the library. I just wanted to know that :

In reduce method at line number 89 - 91(

pyddlib/pyddlib/dd.py

Line 89 in b506d53

if isinstance(key, Number) and isinstance(oldkey, Number):

)
why for the comparison of two keys, we are subtracting two numbers (if key are numbers).
Can you please help? Since In Bryant paper it just says to compare.