That's typical with a triple chainring up front. In the bottom, small ring, you only have larger sprockets in the back, and in the large ring up front, only the smaller sprockets in the back. I seem to remember seeing a video about this sometime recently. It was on one of the sponsors of this site--the bicycle mechanic buy. He has a video, I think, on shifting or adjusting dérailleurs. I'll look for it, and post it here if I can find it.

Edit: Found it: http://bicycletutor.com/gear-shifting/

BTW, I suppose I should qualify what I said. If your bike has a triple chainring, but is set up for half-step with a granny gear, the only time you will have problems is if you are in the granny gear (small one up front) and try to go to the smaller ones in the back. I'm no expert, but I think you can tell a half-step system because the two larger gears will be much closer in size to each other than to the smaller chain ring.

-Jon