Yes, the lighting. I use reveal bulbs these day but still happens at times especially when zooming in or out.
You'e right about how a zoom affects the exposure/colour, A2 - as do the different artificial light sources. I take clinical photos of my patients and the technical steps in the lab and aim for uniformity throughout the case/between patients with the dept. SLR and I find that the zoom can affect the colour, which is supremely important in my presentations as colour matching is what people want to see. Even the background can affect the appearance of the subject.