You‘re mistaking „chip“ for CPU. Every new Apple Watch got a new „chip“, i.e. SiP, even if the CPU wasn’t always updated. (The CPU was updated yearly up to the S4, then again for S6 and S9.) Because of this - and other reasons - it is very misleading to call subsequent iterations „basically the same watch“. There were hardware and feature improvements with every generation of the Watch.
Details - Schmetails. S1 to S10, U1 to U2/3. All the exact same watch, no differences at all.