Group data affecting Sum total of unique count
I'm getting some weirdness with a report I'm working on. I've created a second report which as a sense/sanity check but looking at a cross section of the data where I spotted the anomaly. If I do a distinct count of booking ID against the dates of sessions filtered to a particular month, I get a total of 792. If I then group the dates into months ( to make the report easier on the eye) , for example October for a specific year (matching the exact range of the filter), The number changes to 738. There aren't any rows left out by the group as they'd be listed below or above the row for the group. How can the data change to noticeably when it's the same data but just grouped together? Has anyone else come across this?
Also I don't know which version I'm using, so I've made a best guess from the drop down