What dictates the values that are displayed as aggregations at the bottom of the Opportunity Grid?

Aggregate totals that are shown at the bottom of the opportunity grid columns will reflect either the aggregation of the column or the aggregation of the selected opportunities based on the selected aggregation amount.

Aggregates are generated for currency and number fields that are on the defined query object in the selected mode from Field Config.

The screenshot shows the different currency/number aggregations and the colours show the currency field linked with the aggregation. 

Where there is a checkbox field the aggregation at the bottom of the column will be generated based on the Total field selected in the top right. The aggregation displayed will be either the total of the checked (default) or unchecked selection. So, as per the screenshot, the aggregate in the checkbox column will be aggregated ACV value from the checked opportunities.