How large is the change? A change by inclusions or also, for fully pure growth, the crystal structure is rather plausible.

There may be a few possibilities. High rpm leads to higher centrifugal force, which leads to thinner film, in which quantum confinement effects may exist, which can alter the electronic structure and, thus, the bandgap.

If you double the rotation speed without doubling the deposition rate, your film will be very different.