A single fluid approximation which treats perturbations in baryons and dark matter as equal has sometimes been used to calculate the growth of linear matter density perturbations in the Universe. We demonstrate that properly accounting for the separate growth of baryon and dark matter fluctuations can change some predictions of structure formation in the linear domain in a way that can alter conclusions about the consistency between predictions and observations for ΛCDM models vs modified gravity scenarios. Our results may also be useful for 21 cm tomography constraints on alternative cosmological models for the formation of large scale structure.