https://docs.python.org/3/reference/lexical_analysis.html#indentation reads

Point 1:
"Tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight"

and in the next paragraph

Point 2:
"Indentation is rejected as inconsistent if a source file mixes tabs and spaces in a way that makes the meaning dependent on the worth of a tab in spaces"

In point 1, each tab has definitely a unique space counterpart, in point 2, tabs may have different space counterpart, which one is reliable ?

The documentation should state that Point 1 concerns cPython, or at least indicate that the 8 may depend on the implementation, which then gives sense to point 2.