The limits the material editor is displaying are incorrect. I will fix that up on our end.
The reason is that the engine has to use some samplers internally for lighting features. If your material is opaque you should have 13 samplers available. For static lighting, we reserve two for sampling from lightmaps and one for sampling from the distance field shadow map. For dynamic lighting, we reserve three samplers for reading from the indirect lighting cache. For translucent lighting we reserve 2 sampler for reading from the translucent lighting volume if you’ve selected non-directional and 4 samplers otherwise.
In UE3 it was a bit simpler, you had 12 samplers for static lighting and 15 for dynamic lighting.