Shapes.AddCanvas center of the page and bigger width

I need to add two line in the page first I have written a small function add canvas on word page. the canvas wider than page width 17cm data following

.PageWidth = CentimetersToPoints(21) 
.PageHeight = CentimetersToPoints(29.7)
.LeftMargin = CentimetersToPoints(2.8)
.RightMargin = CentimetersToPoints(2.6) 

and my code following

Dim sht, shb, lineT1, lineT2, lineD1, lineD2
        Set sht = .Shapes.AddCanvas(CentimetersToPoints(2), CentimetersToPoints(5), CentimetersToPoints(18), CentimetersToPoints(1))
        Set shb = .Shapes.AddCanvas(CentimetersToPoints(2.54), CentimetersToPoints(16.8), CentimetersToPoints(17), CentimetersToPoints(1))
        Set lineT1 = sht.CanvasItems.AddLine(0, 0, CentimetersToPoints(17), 0)
        Set lineT2 = sht.CanvasItems.AddLine(0, 2, CentimetersToPoints(17), 2)
        Set lineD1 = shb.CanvasItems.AddLine(0, 0, CentimetersToPoints(17), 0)
        Set lineD2 = shb.CanvasItems.AddLine(0, 2, CentimetersToPoints(17), 2)
        With lineT1.Line
            .ForeColor.RGB = RGB(255, 0, 0)
            .Weight = CentimetersToPoints(0.05)
        End With

        With lineT2.Line
            .ForeColor.RGB = RGB(255, 0, 0)
            .Weight = CentimetersToPoints(0.02)
        End With

        With lineD1.Line
            .ForeColor.RGB = RGB(255, 0, 0)
            .Weight = CentimetersToPoints(0.02)
        End With

        With lineD2.Line
            .ForeColor.RGB = RGB(255, 0, 0)
            .Weight = CentimetersToPoints(0.05)
        End With

why in the word the canvas display 15.86cm not 17cm,and postion not center of the page enter image description here