CQ for Downsampling - Timestamp seems odd (offset to interval start)

I came across a really odd behavior of Continues Queries used to downsample data. The new, downsampled data seems to get a timestamp which is the start of the time-internval that was downsampled. This is quite counter-intuitive as one would expect to have the data timestamped to the middle of the interval.

Did I misunderstood something and how would I need to change CQs to be correctly timestamped?