I would guess that when you say you turn the TV off you are using the remote control to do so, not the switch at the wall.
Most TV's don't actually turn off with the off button, they just go into standby mode. This allows them to monitor your remote control so that they can sense when you command the TV to turn back on again via the remote.
In the same way, many TV's will monitor input signals (HDMI, Scart etc.) and turn themselves on when a new signal is received. I suspect that what is happening is that when the Raspberry Pi is coming out of sleep, it starts sending a video signal to the TV again, bringing it out of sleep too.
Alternatively, as Krzysztof suggests, Raspbmc may be commanding the TV to turn on via the CEC protocol channel, so you may be able to disable it.
According to the HDMI CEC and XBMC thread on the Raspberry Pi forums, the libCEC FAQ explains how to do this:
Where can I find the CEC adapter's settings in XBMC?
- Go to System -> Input -> Peripherals -> Pulse-Eight CEC Adapter.
What's the meaning of the CEC adapter's settings in XBMC?
- Enabled: use the CEC adapter within XBMC. Default: Enabled.
Of course while the upside of off meaning standby is convenience, downside is that the TV consumes power even when it is off.