As heero-yuy says newer sets support the reception of other multiplexes. T2 multiplexes don't just carry HD channels, they can also carry Standard Definition channels but more modern technology means they take up less space and more can be fitted in. The T2 multiplexes in the UK carry a mixture of HD and SD content.
Also if you live in the reception area of a Local TV Channel you can receive there are extra Sony-owned channels and a shopping channel.
A breakdown of the multiplexes are here.
http://www.xhxf.uk/timeshares/ It's mostly accurate, but doesn't have the shopping channel on the Local multiplex listed.
Also, not all TV tuners are the same sensitivity as each other so this could also be a factor in which multiplexes you are receiving, despite them sharing an aerial. Also, cabling can play a factor, length, quality of cable, shielding from nearby interference.
Heero-yuy, "HD Ready" just means the screen support high resolution inputs and can display them e.g. from an extra TV box or a games console, it doesn't mean it has an HD TV Tuner inside. The "HD Ready" branding was around before HD was broadcast on Freeview.
I wonder if the Mods could move this thread to the Freeview section, it's nothing to do with IT.