This gets down to the heart of frequentist statistics - how do we estimate the true value/measure of something? Of course, the easy way to get an idea is to look at data, but data never represents the true value, it can always fluctuate. So what do we do to measure our Rubik's Cube times? We take "averages;" specifically, truncated averages, as we always drop the best and worst X%, depending upon the average.
I see many people define it as an expected value, but this is NEVER done in practice. First, how do we deal with DNFs in expected values (when we take an average of all the solves)? Even in our sample averages, we usually take out the best and worst X solves. When we do this trimming, this is taking the true definition of average (not the cuber definition) and moving it more towards the median. The median is actually a trimmed average, although it's a bit trivial since it's doing the maximum amount of trimming. Cubing is also a very luck-driven activity, especially on the bad luck side of things. Pops, lockups, wrong algorithms, there's so many things that can go drastically wrong, and this is often why we employ the trimmed average, as this allows us to make a mistake without shooting ourselves in the foot. It also better reflects what our usual time is, as the average would be typically a bit higher than our usual time with our terrible solves pushing the average up. So taking an average or expected value of your times isn't a good approach, as it doesn't get our usual times, and it doesn't account for DNFs.
Also, looking at a sample average isn't a good way either, as this can vary greatly depending upon the time of day, amount of warm up solves, etc. We want to get our true skill level, and no matter how many solves you do, you can never really get the exact true value, although you will get close. Thus, I think it's best to look at global "averages" truly as global medians. Ask yourself this: what time can I anticipate to beat 50% of the time? I'd think that best describes your solving skill the most out of any measure.