it's a leap year issue, last year was a leap year so in the 1 year prior data column, 1 day gets slashed from the uptil month which you pull the data for. As you keep on adding more months to the column, the previous months keep getting fixed and upcoming month loose data worth a day! Quite interesting though, thnks. Hope this might help someone else.
We noticed this with the '1 year prior' column and have notified it to engineering. However, since it is affecting a column which not many customers use, and has easy workarounds, it is set to low priority.
"when i look out of my window, i can see 99 sheps. compared to yesterday, there is one less! same as two days ago! might it be due to the bad wheater condition?"
i really tried hard to understand your question, but there might be so many reasons for a difference. can you write a little bit more about the problem? do you expect something different? did you have other numbers a year ago? do you use any segments? ...