HeapTupleHeaderData Layout description

‌Hi,

In https://www.postgresql.org/docs/13/storage-page-layout.html
we can find on § 68.6.1. Table Row Layout
the Table 68.4. HeapTupleHeaderData Layout

When additionning Length of differents Fields taht are present we obtain  atotal of
4+4+4+4+6+2+2+1 = 27

What doesn't seem to fit with previous indication "There is a fixed-size header (occupying 23 bytes on most machines)"

Researching in old doc shows that in 8.2 the documentation
https://www.postgresql.org/docs/8.2/storage-page-layout.html
indicate 27 bytes (with the same detailled Table)

Number 23 appears from Postgres 8.3

In some thread like https://stackoverflow.com/questions/2966524/calculating-and-saving-space-in-postgresql/7431468
It's possible to read that
Overhead per tuple (row)
... And at least 24 bytes (23 + padding) for the tuple header ...

As there is several thread about this point I suppose there is an evolution between 8.2 and 8.3 with a missdocumentation for an evolution of detailled Table "HeapTupleHeaderData Layout"

IS it right ? And in this case what is the real detailled Table ?

Thanks for any explanations
Regards