The Distance Interval with which the Template Cube was created may vary slightly from the resulting Output Space Time Cube.
If you created the Template Cube prior to ArcGIS Pro 2.1 with a user-defined Distance Interval using the unit of measure feet, the unit feet used was International Feet. This has changed with the release of ArcGIS Pro 2.1, which has been updated to use the unit US Feet. This will cause slight differences in the grid cell size between the Template Cube created using International Feet and the Output Space Time Cube using US Feet.
However, if the spatial reference of the Template Cube used US Feet, there will be no change.
Re-create your Template Cube using ArcGIS Pro 2.1.