.net - Why Is Graphics.DrawLine Drawing A Wedge Shape

I am trying to draw a bus route as a simple sequence of lines. Nothing fancy. But instead of lines, I'm getting wedges. Initially I was fine with this, because the wedges sortof look like arrows and always face towards the second point. But now I want to improve the look, and the wedges are becoming a big problem.

My suspicion is some sort of floating-point issue due to the graphics transform (lat/lons are fed in and the transform turns them into x/y on the bitmap [assuming lat/lon is euclidean is accurate enough for my purposes], so the scaling is several orders of magnitude).

Screenshot:

It actually kind of looks like the line was split into two triangles but only one of them was drawn.

Relevant Code (note: drawing is done asynchronously, which is why I'm creating a bitmap):

'-- Creating the transform --'
Dim bitmap = New Bitmap(Math.Max(1, PictureBox1.Width), Math.Max(1, PictureBox1.Height))
Dim g = Graphics.FromImage(bitmap)

[Code].....

Using DrawLines instead of DrawLine solves the issue (but why?) Increasing the pen thickness makes the issue go away (but the lines are too thick) Zooming out (increasing lat/lon view window) makes the issue go away, eventually (but I want to zoom in!)