Why has new bookwork installation given me a newer kernel than an upgraded bookworm?

I'm confused, I've just installed Raspberry Pi OS (Bookworkm) on a new Pi 4B and
it has installed Kernel: 6.6.62+rpt-rpi-v8 aarch64.

I have another 4B on which I installed Bookworm a while ago and have
updated regularly and it only has Kernel: 6.1.21-v8+ aarch64.

Why hasn't the older (to me) system got the latest kernel as installed
on the new one?

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Chris Green
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