[pdftex] magic height 1.00376129150390625pt for coloring the rules

[Stanis³aw Romañski]

Hello,

Is there something magic in the dimension 1.00376129150390625pt for pdfTeX ?
The rules of smaller height seem not to be colorable (and the higher ones,
yes).
The simple code

\pdfcompresslevel=0  \nopagenumbers
\pdfliteral{0 0.4 0.1 0 k} % Pink
\hbox{\vrule height 1.00376129150390625pt width 15pt depth 0pt}
\hbox{\vrule height 1.003761291503906249999999999999pt width 15pt depth 0pt}
\bye

gives two rules: the upper is pink and the lower is set black. Why ?
My tests show this does not depend on rule width; \hrule behaves the same
way.
I am using pdfTeX, Version 3.14159-14g-pretest-20000912 (Web2c 7.3.2.2)
Any suggestions how to fix it or go around ?

Stanislaw Romanski