If you clear your workspace, so that samplemean is NOT defined when you start to run the loop, then it will look as you expected. However, IF you run that loop repeatedly, WITHOUT a clear in between runs, then samplemean is already defined as a vector of length 5, and it is full of numbers in advance.
One thing to take from this is that clearing your workspace can sometimes be a good thing. It is often a very bad thing too of course. So you are best off learning to use and write functions, which have their own workspaces. One thing at a time though.
(By the way, I'm quite happy to see that you are reading the tutorials and thinking about what you see, as well as trying out the examples.)
