Whenever you use “today” in comparisons, the corresponding day is compared up to the last hour for which today's data is available. This gives you a more useful comparison value since same periods are compared.
You can now use any raw data metric or formula for a meaningful comparison of today with yesterday or another day.
KPI Analyses (comparison variant)
If “today” is included, the corresponding day is compared up to the same hour of the last data update of today. So you compare "today until 11 o'clock" with "yesterday until 11 o'clock", which gives you a meaningful result.
In sparklines, the trend is more relevant, so the total time range compared is not limited to the hour of the last update for that day.