Basically what I’m asking is why is the century date different from the century “number” if you understand what I mean
Year 1-99: first century Year 100-199: second century And so on
To have the nineteenth century correspond to the 1900s, you'd need to have a century zero (or zeroeth certury).
Edit: This answer isn't lengthy or in-depth, but I don't think there's a more complex answer. I'd love to hear it if there is.