These two seem to be at odds.  Is this the case?

>From help(factor) - section Warning:

To transform a factor f to approximately its original numeric values,
as.numeric(levels(f))[f] is recommended and slightly more efficient than
as.numeric(as.character(f)).

>From the language definition - section 2.3.1:

Factors are currently implemented using an integer array to specify the
actual levels and
a second array of names that are mapped to the integers. Rather
unfortunately users often
make use of the implementation in order to make some calculations easier.
This, however,
is an implementation issue and is not guaranteed to hold in all
implementations of R.

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