It was probably upgraded from ~30A at some point in the past as a requirement of a lender. Presumably it has circuit breakers? Those weren't even a choice in a 100 year old house when it was built AFAIK. I've been curious how well these jobs are generally done, when I was a kid it was a big selling point to have circuit breakers instead of fuses. I've always been curious how through some of those upgrades were, did they replace all the wiring too, does everything have grounds, etc. Or did they just pull a few 240 lines for AC/range/etc?
https://www.thespruce.com/service-panels-changed-in-the-1900...
Which says 100A didn't become common until the 1960's.