Rescaling during support mapping unclear

[bmichanderson]

Hi, I'm trying to map bootstrap support from an SVDquartets analysis. I have 100 bootstrap trees in a Nexus file (boots_sp.tre) and I want to use them to map support onto my inferred tree (qfm_92_sp.tre). Here's the command I am using with IQ-TREE v. 2.4.0 for Linux x86 64-bit:

iqtree -sup qfm_92_sp.tre -t boots_sp.tre

It runs fine, but shows this output:

Reading tree qfm_92_sp.tre ...
unrooted tree detected
Reading input file boots_sp.tre...
100 tree(s) loaded
0 split(s) discarded because frequency <= 0.000
9 taxa and 23 splits.
Rescaling split weights by -33.333
23 splits found
Creating bootstrap support values...
Tree with assigned support written to boots_sp.tre.suptree

In previous runs on larger datasets (more than the 9 taxa here), I had to add -scale 1 to switch to percentages (IQ-TREE default is 100* the value). But in this case, it is dividing all support values by 3 for some reason I can't understand. I can correct it by setting -scale 3, but I'm unclear why and how to predict that value beforehand.
Does anyone know what might be going on?
I'll attach the relevant files (renamed to *.txt for upload) for replication.
boots_sp.tre.suptree.txt
qfm_92_sp.tre.txt
boots_sp.tre.txt