Consider this graph from Query Performance Insight:
What's interesting about this image is that the green line shows an overall max Data IO metrics of about 20%, while the summed query Data IO is only at 0.08%.
What exactly is the reason for this discrepancy? I know that we have a query that only sometimes causes high Data IO, the rest of the time it doesn't, but that shouldn't get lost when aggregating as "sum", right? Or are averages calculated somehow despite choosing "sum" as the query aggregation?
I'd really be interested to understand this in detail.