How does my just-booted and encrypted iPhone still receive messages before I decrypt it with the passcode?
Probably because you didn't just reboot it. All you did was turn on the screen.

Once you enter the passcode, some but not all of the data remains accessible until you turn the phone (not just the screen) off. To turn the phone off, press and hold the power button for 5 seconds until a "Slide to turn off" slider appears, and slide it.