please check this

owner@localhost:~/math/cuboids> gp
Reading GPRC: /home/owner/.gprc
GPRC Done.

                                          GP/PARI CALCULATOR Version 
2.18.1 (development git-1fa1ee045f)
                                           amd64 running linux 
(x86-64/GMP-6.3.0 kernel) 64-bit version
                                              compiled: Mar 29 2025, gcc 
version 13.3.0 (SUSE Linux)
threading engine: single
                                                  (readline v7.0 
enabled, extended help enabled)

                                                      Copyright (C) 
2000-2025 The PARI Group

PARI/GP is free software, covered by the GNU General Public License, and 
comes WITHOUT ANY WARRANTY WHATSOEVER.

Type ? for help, \q to quit.
Type ?18 for how to get moral (and possibly technical) support.

parisizemax = 10000003072, primelimit = 1048576, factorlimit = 1048576
? chkr1=(a^2-b^2)/(2*a*b)
%1 = (2*a^2 - 2*b^2)/(4*b*a)
?
What??? where did that extra factor of 2 come from?

While %1 is correct, in extracting the numerator and denominator, things 
quickly fall apart.