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The epsilon phase precipitates out first. The precipitates are visible in the micrograph as the phase surrounded by the paler eta phase.

When the system goes below the peritectic temperature the epsilon phase starts to transform to eta and new eta phase begins to nucleate heterogeneously on the surface of the epsilon precipitates

The peritectic reaction (a solid plus liquid transforming to another, different, solid) does not go to completion due to the new phase creating a barrier to diffusion.

This prevents the composition of the system homogenising, and so the remaining liquid phase is not of the bulk composition, and so there is less eta than predicted by thermodynamics.

This micrograph is of the same magnification as the sketch. See also the DoITPoMS micrograph library entry..

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