Imagine this toy example. You have 5 series, which are very sparse, as is often the case in retail. For example, series 1 has sales on Mondays and 0's the rest of the days, series 2 on Tuesdays, series 3 on Wednesdays, and so on. For those individual series, a value close to 0 would be more or less accurate, however, when you add all the predictions up, the value will be way below the true value.