Partially-fluorinated mesogens exhibit some of the most significant de Vries behaviour (anomalously low layer shrinkage at the SmA–SmC transition) reported experimentally, making them of particular interest for technological applications. Force constant parameterisation and subsequent atomistic simulations of the mesogen 3M 8422 enabled experimental trends in layer spacing, tilt angle and order parameter to be successfully replicated in silico. Analysis of the simulations demonstrated inconsistencies with conventional models of de Vries behaviour, but analysis of the hydrocarbon and fluorinated sub-layers showed clear evidence that contraction of the hydrocarbon sub-layers was offset by simultaneous expansion of the fluorinated sublayers. Most notably, the behaviour was entirely consistent with that recently reported for the non-fluorinated mesogen 9HL, in which a sub-layer remains “SmA-like”, even in the SmC phase. This observation is somewhat at odds with the relatively established theory that it is specifically nanosegregating elements that promote de Vries behaviour, and suggests there is much still to learn about the molecular organisation within these systems.