I've noticed there is not an option to make the cell size for both X and Y the same when interpolating point data in QGIS 3.0.
When adjusting the cell size in QGIS 2.18, the columns would automatically adjust. Someone once told me the more ocean that is in the frame, the more askew the resultant interpolated raster will be, unless the cell sizes are set the same.
Below is a screen shot from 2.18
And from 3.0
This is an important feature for me because if the resultant raster pixels end up rectangular instead of square, any viewshed analysis I perform end up some 15-50km to the north of my area of interest.