The BCM2711 SoC inside is a newer stepping (C0 vs B0 in Pi 4 model B), and can run faster and slightly cooler than the older revision.
Thus the Pi 400 has a 1.8 GHz default clock, while the Pi 4 model B has a 1.5 GHz clock.
Note that I just checked my Compute Module 4, and it also has the C0 revision of the chip... so it's interesting the Pi Foundation chose to keep it at the default 1.5 GHz.
It might have to do with the fact that the Pi 400 has a massive keyboard-sized heatsink attached to the SoC in the unit!
Thus the Pi 400 has a 1.8 GHz default clock, while the Pi 4 model B has a 1.5 GHz clock.
Note that I just checked my Compute Module 4, and it also has the C0 revision of the chip... so it's interesting the Pi Foundation chose to keep it at the default 1.5 GHz.
It might have to do with the fact that the Pi 400 has a massive keyboard-sized heatsink attached to the SoC in the unit!