Yes this is very normal, as some Wii games don't conform to true 16:9 in 480p. ExciteTruck has thin black borders, but the newer ExciteBots does not. Gravity is very bad, with huge black bars on both sides. I find most Nintendo games such as SM Galaxy, MarioKart, etc. fill the entire screen perfectly. When I first noticed the bars, I thought maybe a setting got changed on my TV, but it's just how it is. Another strange occurance for me is the Super Nintendo (Snes9xGX), where most games fill my 16:9 when I get the settings dialed in, but then some games, namely Street Fighter II World Warrior have their own funky aspect ratio.
I just find that owning a 16:9 hdtv is going to result in a mixed bag of compatibility, considering a lot of media just wasn't designed with this format in mind.
It's probably to do with overscan. TVs often don't display the edges of the picture. It was originally for tech reasons to do with old CRTs, but it hasn't really been necessary for years - they just do it because a lot of media is designed for it.
Games programmers want to squeeze as much performance out of the system as possible, so sometimes they just don't render anything at the very edges since it's going to be overscanned (cut off) anyway. They don't do it anywhere near as much these days since there are plenty of HDTVs that don't overscan at all on progressive scan inputs.