In Analytics, you can set your date filters to use specific dates (e.g. 9/9/2021 - 9/10/2021), or use sliding dates (e.g. Yesterday - Today). Sliding dates are always relative to the current day, whereas specific dates are static and always reference a specific point in time.
When you e-mail a dashboard, sliding dates are always converted to specific dates.
We do this because when you e-mail a dashboard, you are sending a copy of what that dashboard looked like at that point in time. The data will always reference what it was set to, at the point the dashboard was sent out. This is true not only of the data but also of how the dashboard is set up. Let's say you e-mail a dashboard to someone, and then right after e-mailing it, you decide to add another widget. The e-mailed dashboard would not include the new widget you added. Think of it this way. E-mailing a dashboard is like exporting it to PDF and emailing the file to someone. The PDF is a snapshot of the dashboard, and you wouldn't expect the data or the layout of the dashboard to change every time you open the PDF from your e-mail.