Posts Fading Background Issue

I thought it would be cool to have the posts' backgrounds fade into nothing, as the posts get older. I came up with this bit of CSS...

#posts .box:nth-of-type(2) { background-color:rgba(255, 255, 255, 0.9); }
#posts .box:nth-of-type(3) { background-color:rgba(255, 255, 255, 0.8); }
#posts .box:nth-of-type(4) { background-color:rgba(255, 255, 255, 0.7); }
#posts .box:nth-of-type(5) { background-color:rgba(255, 255, 255, 0.6); }
#posts .box:nth-of-type(6) { background-color:rgba(255, 255, 255, 0.5); }
#posts .box:nth-of-type(7) { background-color:rgba(255, 255, 255, 0.4); }
#posts .box:nth-of-type(8) { background-color:rgba(255, 255, 255, 0.3); }
#posts .box:nth-of-type(9) { background-color:rgba(255, 255, 255, 0.2); }
#posts .box:nth-of-type(10) { background-color:rgba(255, 255, 255, 0.1); }

Unfortunately, the first and second posts are getting the backgrounds switched. I'm not sure why, because Internet Explorer's and Firefox's developer tools are showing the code the way it should be (second post being 0.9 opacity), but they look backwards.

It's very queer, but it's still a cool effect (once you move past the stupid part).