Where does paper-height come from?

[b1]

Hi all,

I am playing with the vertical spacing algorithms, and am very surprised by the 
behavior of "page-height". 

Let's say for example that I set my paper to A6.  This, by ISO216, is 148mm 
high. 
Following the code in paper.scm, (I am using the most current git head), I can 
see inernal-set-paper-size calling (set-paper-dimensions m w h) with h=148.  
This in turn calls (module-define! m 'paper-height 148).  So far so good.  Now 
the shocking part:  when calc-printable-height applies (ly:output-def-lookup 
layout 'paper-height), it ends up in Output_def::lookup_variable() and 
ultimately in ly_module_lookup().  This does not return 148, but 84.22!!!  Ok, 
the debugger does tell me it's a different module.  But who binds 'paper-height 
to 84.22??  I textually grepped the whole tree for the symbol, paper-height, 
and there seems to be no other place where it is set.  Am I blind, or out of my 
mind?