I started a new world with the server for 1.7.2 and did not specify a seed in the server.properties file. Later, when 1.7.4 came out, I updated the server to the new version and for some reason the seed changed at some point. I checked my first 1.7.4 backup and the seed was the same as the initial seed. When I checked my 2nd backup from a few weeks later, I found that the seed was different. I first noticed it after I had upgraded the server to 1.7.5 where I have found that the seed is different from either of the two previous seeds.

I double checked and created a single-player world with each version with the original seed and noted that the world stays the same for each version. I'm not sure where the seed is stored or how/why it would be changing but this is not expected behavior. When I've created server worlds in the past, I never noticed the seed changing. I have never used a pre-designated seed in the server.properties file but I assume if I had, the seed would be the same for all of them.