archive log size in 11g

I notice some thing different for my archive log size. For the previous versions, when I set my redo log to be say 60 mb, the archive log size would usually be consistent in size. For this 11g version, I set my log size to 30mb, but each archive log size seems to be different some at 27033088, 29011456, 26030080, 25472512, 30339584, just to be an example, not really uniformed in size like it use to be in the previous version. I am just curious is this the new behavior with the 11g? I don't know how to explain this.