As said, it is not ideal to be at such a small scale while using the drawing tools. You can then downscale the model just before sending it to the printer.
SketchUp has been created long before it was found useful for 3d printing. Of course nowadays when designing a new program, one would probably take different decisions and finetune parameters differently.
When you draw an edge, a lot more happens then just storing two points: There are these features that make modeling in SketchUp so special and convenient: snapping, automatic edge merging and breaking (instead of allowing to overlaying geometry), and the internal tolerance (to detect floating point coordinates with rounding errors as being the “same”). This comes with little performance cost and issues at small sizes. In your case the edge was falsely recognized as overlapping the neighbouring edge, and SketchUp broke it and merged it with the nearby vertex.