Because PureOS Amber is/was related to Debian Buster (as LTS from July, 2022 to June, 2024) and while PureOS Byzantium is currently related to stable Debian Bullseye, precisely to 11.2 version/distribution of it.
In order to get there where you want regularly you’ll need to wait for the next PureOS edition/release (for sure) or in order to get more recent Linux Kernel as of today and out-of-the-box you’d need to consider of moving your /home on top of Debian Bookworm repos (or some other testing distribution). Would you please correct you title to amd64 related Kernel:
uname -r
5.10.0-10-amd64
lsb_release -a | grep Codename:
…
Codename: bullseye
P.S. Also we are not extending your thread topic to byzantium-security/ packages either, no need.