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"Average" is a widely misused word in English. There are three averages you might be talking about when you use it - mean, median and mode. Most of us will mean "mean average" when we use it though, and that's what the time-in-status means.
Consider a sequence like 1, 1, 3, 7, 21 - mean = 6.6, median 3, mode 1.
They all have different uses and tell you different things about your data set. When used together, they can tell you a lot more. For example, if you look at the "average UK income", the mode won't tell you a lot, but looking at the mean and median immediately screams that there's a massive rich/poor gap.