experiencenub.png is a texture located in the textures/ui directory. It is the texture that separates the segments of the experience bar.

Unlike most textures, this one does not seem to scale. If you provide a texture that is not 11 by 5 pixels then the texture will not be rendered correctly.

I have attatched "experience bar broken.mcpack" which uses an "experiencenum.png" texture with the dimensions 44 by 20 pixels. You can see in the attatched screenshot "broken.png" that this does not render correctly.

I have also attatched "experience bar working.mcpack" which uses the default dimensions for the "experiencenub.png" texture. You can see in the attatched screenshot "working.png" that the texture renders correctly in this case (as the default dimensions are used).