That's because your Amiga is still a PAL Amiga. Changing to NTSC through early-boot-menu is a way of making your PAL amiga run still in PAL, but not @ 50hz anymore but 60hz. You are running the so called PAL60.
The same thing would happen to an NTSC A1200 that tries to switch to PAL mode from early-boot-menu, except it would work in NTSC50 mode.
The PAL/NTSC multi-standard of Amiga computers is a very relative term. Deep down they are locked to either NTSC or PAL. Amigas with TV tuners such as A600 and A1200 take this to another level where the TV tuner itself is also either PAL or NTSC specific.
