3D print tools: how can "too thin" warning be only on 1 face? Using 2.70a on Windows7. Got a nice Ultimaker 3D printer to play with 

Editing my mesh of a cartoon character to print on the Ultimaker. Blenders 3D print toolbox mesh analysis looks really useful. I've watched the BF video course and that's excellent too. (Thanks Dolf!)

I find that the 'Thickness' overlay mysteriously colours isolated faces with red/green/etc to warn me they are too thin to print (e.g. less than 2mm) but many of these faces have lots of depth behind them, so this really doesn't make sense to me.

Stranger, I've seen tubular parts being given just one face with a colour warning, but the directly opposite side of that mesh remains uncoloured. Surely thickness is the distance between opposite faces, so these warning colours must go on opposing pairs of faces, right? How can only one show as a problem?

I've experimented with a simple cube by sliding just the top-front edge backwards (creating a wedge) until colour warnings appear on any faces. I find that the warning first appears only on the bottom face. As the edge advances towards the rear the bottom deepens in colour until the rear face triggers a warning too. Then later the sloped front face gets its own warning colour. For the life of me I can't work out its logic.

Understanding this tool's logic is important because the mesh edits we need to carry out to solve these thinness problems are governed by how it thinks!

Any ideas?

TIA,
Slim
The thickness measurement uses ray-casting to find faces which are close together.

One thing to consider is the thickness is measured along the direction the face is pointing, so in some cases you could have shapes which give different results with wrinkled vs smooth surfaces).

In general, increase the number of samples for a higher quality result.