Yeah, it's a little confusing.  Basically, the `SmallArea/tilemapresource.xml` file used in the test specifies that the first level is level 11, it uses the Web Mercator tiling scheme, and that the overall extent of the imagery is as specified.  If you do the math on that (you can get `WebMercatorTilingScheme` to do it for you) that means there are 4 tiles at the first level, level 11.

Meanwhile, our terrain, which is the default `EllipsoidTerrainProvider`, has two tiles at level 0, one covering the western hemisphere and one the eastern.  And our test is computing which imagery tiles overlap these two tiles.

All four imagery tiles are in the western hemisphere, so they all overlap the western hemisphere terrain tile.  That's why there are four for `tiles[0]`.  None of them overlap the eastern hemisphere terrain tile, though, so why is `tiles[1].imagery.length` expected to be 2?  That's because the imagery layer is a base layer, so the two tiles eastern-most imagery tiles are stretched all the way to the eastern hemisphere of the globe.

I'm not completely sure if I've answered your question, so let me know if that helps.