There are several things that could contribute to brittle teeth in addition to where on the tooth line you set a tooth.
What can change the structure of steel is the amount of brown oxide ( rust ) or black oxide ( patina ) the saw plate has endured in its lifetime. Any iron that is converted to oxide increases the proportion of carbon in the remaining steel and also makes that steel more like a sponge or Swiss cheese, making it weaker and more brittle. Take a loupe to an old saw and you may see some of that.
And there is the possibility that older cast steel wasn’t always as uniform as modern steels to begin with. One of the processes a saw plate used to go through was hammering to tension the plate near , but not on, the tooth line. What this did was work harden ( paul calls it “consolidating” when he talks about card scraper sharpening ) parts of the plate that eventually became part of the tooth line with repeated sharpening. Generally, the better saws had more of this hammering done than cheaper plates because it was skilled labor intensive. A study of old Disston saws reported no difference in the steel between their saws but reported their “extra refined steel” likely only referred to the amount of tensioning in more expensive saws.
One saw company, Simonds, stopped making handsaws around 1923 over a labor dispute about the relative higher pay of these specialists compared to other workers who unionized.
I don’t think modern factory produced saw plates suffer from any work hardening-tensioning as part of the saw making process, and probably haven’t since at least WWII. At least I haven’t seen any pictures of gangs of workers hammering plates with hammer and anvil since the 20’s or 30’s. Later sawmakers either used more complex steels or ignored the issue altogether. Modern induction hardened saws have never seen a hammer. Only modern “boutique” sawmakers might do some tensioning, and if they do, they are sure to let you know it.