Inconsistent results with Marshal.ReadInt32.

The title may be confusing. Let me explain. I have the following method:

vb.net Code:

Sub SpeedStitchTest()
        Drawn = False
 
        bmpData = bmpMap.LockBits(New Rectangle(0, 0, bmpMap.Width, bmpMap.Height), Imaging.ImageLockMode.ReadWrite, Imaging.PixelFormat.Format32bppPArgb)
 
        Dim data As BitmapData = bmpImage.LockBits(New Rectangle(0, 0, bmpImage.Width, bmpImage.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppPArgb)
        Dim AlphaInt As Int32 = Alpha.ToArgb
        Dim ImageColorInt As Int32 = 0
        Dim MapColorInt As Int32 = 0
 
        Dim ShortCircuit As Boolean = False
 
        Dim sw As Stopwatch = Stopwatch.StartNew
 
        For CanvasY = 0 To bmpMap.Height - bmpImage.Height
            For CanvasX = 0 To bmpMap.Width - bmpImage.Width
 
                If Drawn Then
                    Exit For
                End If
 
                MatchCount = 0
                PixelCount = bmpImage.Width * bmpImage.Height
 
                For nY = 0 To bmpImage.Height - 1
                    If ShortCircuit Then
                        ShortCircuit = False
                        Exit For
                    End If
 
                    For nX = 0 To bmpImage.Width - 1
 
                        ' Read image pixel.
                        ImageColorInt = Marshal.ReadInt32(data.Scan0, (data.Stride * nY) + (4 * nX))
 
                        ' Discount alpha pixels.
                        If ImageColorInt = AlphaInt Then
                            PixelCount -= 1
                        Else
                            ' Read map pixel.
                            MapColorInt = Marshal.ReadInt32(bmpData.Scan0, (bmpData.Stride * (CanvasY + nY)) + ((CanvasX + nX) * 4))
 
                            ' Discount alpha pixels.
                            If MapColorInt = AlphaInt Then
                                PixelCount -= 1
                            Else
                                ' If the pixels match, increase the match count.
                                If ImageColorInt = MapColorInt Then
                                    MatchCount += 1
                                Else
                                    ShortCircuit = True
                                End If
                            End If
                        End If
 
                    Next
                Next
 
                ' Check the match %.
                HighestMatch = Math.Max(HighestMatch, (MatchCount / PixelCount) * 100)
            Next
        Next
 
        MessageBox.Show(CStr(sw.ElapsedMilliseconds))
        MessageBox.Show(CStr(HighestMatch))
 
        HighestMatch = 0
        bmpImage.UnlockBits(data)
        bmpMap.UnlockBits(bmpData)
 
    End Sub

Both images I use (bmpMap and bmpImage) are from the exact same file (PNG file, 960 x 976). When the method runs for the first time, the match percent is 100, because they are the same image. However, when it runs a second time, immediately after the first time, I get 9.43 (rounded) as a match percent. I am unsure why. To my knowledge, nothing is being changed and the method is running twice, back to back without anything running between. The pixel it gets to is (537, 91), if that matters.

Any ideas what may be happening?