I should explain the current reasoning for the separate time frame logic between global settings and KPIs.
The time frame selection the global settings and the individual graphs' time frame selections are used to control how much data is displayed everywhere but KPIs, because when you select "last 7 days" or "last 30 days" in the KPI dashboard, it's actually selecting the last 14 days or the last 60 days and then calculating the % differences between the last 7/30 days and the 7/30 days before.
The shortest term solution will be to enable the same logic for the "last 90 days" option (since we now have 180+ days of data for all the KPIs, something we did not have when the feature was introduced).
We discussed a separate solution wherein the dashboard intelligently figures out the start & end dates of the current & previous quarters to have a "this quarter vs last quarter" view, but this will require more time to engineer.
I hope that makes it clearer. We will note this in the global settings panel.