The initial estimative of -nt <int> parameter

GROMACS version: 2022.6
GROMACS modification: No
Hi, community.
I would like to know, how gromacs calculate an initial estimate for number of threads used on mdrun?

For example, when run this command “gmx mdrun -deffnm md_0_1 -nb gpu” , for system 1, i can see all my 24 threads been using. Nonetheless, when using a system 2 the -nt parameter is set to 12 threads.

More information:
System 1 is the protein 7aap on pdb database(SARS-CoV2 RNA-dependent RNA polymerase). Running on an water cubic box. Number of atoms: 17746
System 2 is the protein 6lu7 on pdb database( COVID-19 main protease). Running on an water cubic box. Number of atoms: 2500