[pdftex] 1px is not 1bp which seems to contradict the pdftex manual

[jfbu]

Hi,

(please CC if replying)

The pdftex manual says 

The default value of \pdfpxdimen is 1 bp, corresponding to a pixel density of 72 dpi.

I thus expected 1px by default to be as 1bp but it is not:

$ rlwrap pdftex
This is pdfTeX, Version 3.141592653-2.6-1.40.25 (TeX Live 2023) (preloaded format=pdftex)
 restricted \write18 enabled.
**\relax
entering extended mode

*\message{\number\dimexpr10px}
657820
*\message{\number\dimexpr10bp}
657817
*\message{\number\dimexpr1bp}
65781
*\message{\number\dimexpr1px}
65782
*\message{\number\pdfpxdimen}
65782
*\bye

with luatex both 1px and 1bp map to 65781sp.

I don’t know if other engines support px.

I tried also with TL2019 and got same result.

Best,

Jean-François