A: This is most likely due to the date filter applied as standard on the ARR overview dashboard in Report centre:
The date filter is set to Current year (Current fiscal year if your account uses a broken fiscal year). With this filter, Total ARR per month shows the ARR calculated until the last day of that year. Any churn or new subscriptions dated before then will be included in this number.
The Current ARR widget on the same dashboard shows your ARR as of today.
If we check Insights and the report called "Total ARR right now", this shows your current ARR. That's why the value in those two reports can differ.
Insights is an add-on. The ARR overview dashboard is available when your account uses a recurring revenue sales model, see How to get MMR/ARR dashboards in the Report centre?