Width of GRID units does not match width of COLUMNS

[fdb]

I have a grid, created by a BASIC macro, that needs to contain 4 columns. I fail to match the width of the grid with the width of the columns :

const kWidthCol1 = 30
const kWidthCol2 = 50
const kWidthCol3 = 10
const kWidthCol4 = 75
...
	hEventGrid = makeGrid (hDialog, "ttt", 20, 20, kWidthCol1+kWidthCol2+kWidthCol3+kWidthCol4, 150)
	hColumn1 = makeColumn (hEventGrid, "Datum", kWidthCol1)
	... etc

function makeGrid (oDlg, Title, x, y, w, h) as object
	dim oGridControl as object
	dim oGridModel as object
	oGridControl = createUnoService ("com.sun.star.awt.grid.UnoControlGrid")
'	oGridModel = oDlg.createInstance ("com.sun.star.awt.grid.UnoControlGridModel")
	oGridModel = createUnoService ("com.sun.star.awt.grid.UnoControlGridModel")
	oGridModel.useGridLines = true
	oGridControl.setModel(oGridModel)
	oGridControl.setPosSize (x, y, w, h, com.sun.star.awt.PosSize.POSSIZE)
	oDlg.addControl (Title, oGridControl)
	makeGrid = oGridControl
end function

function makeColumn (table, title, width) as object
	dim col as object
	col = createUnoService ("com.sun.star.awt.grid.GridColumn")
	col.Title = title
	col.columnWidth = width
	col.maxWidth = width
	col.minWidth = width
	col.Resizeable = false
	table.model.ColumnModel.addColumn (col)
	makeColumn = col
end function

Resulting in a grid that is far too small :

This suggests to me that the width of the grid is using a different unit than the width of the columns. If so, what is the relation between these two units ?

[fpy]

possible clue from Service UnoControlGridModel