You should be able to see what's in that erg by selecting that erg, then in the "experments" frame select the "API Log" tab and open the log for that erg. My expectation is that this erg is other than a simple "draw" or "clear" command.
Please let me know if this answers your question.
Also is there a known list of costly non-draw/clear calls on the (integrated) graphics? I ask because in some ergs I see more time spent on "Other" than on shader calls.
GPU Breakdown histogram shows per-erg "breakdown" for all the shaders used by the erg and also shows the total time for the erg which includes all other GPU-pipeline stages, like culling, rasterization, blending, etc. Some types of ergs includes only "non shader" stages like Clear call for example. If a DrawPrimitive call has significant "non-shader" part it means that it mostly depends on non-shader pipeline stages.
Here's some additional items that might appear in the "other" category: Vertex Fetch, Input Assembly, Clipping, Rasterization. As Philipp mentions, it's anything in the GPU pipeline that is not attributed to a shader.
Hope this helps...