Although it has been years since I did color printing, I'd agree with Ed's assessment of color-balance variations that might be encountered within a given roll. That is to say, individual shots on the roll might have been exposed under wildly different light, and may be exposed differently. Additionally, as I recall, each batch of paper will have a slightly different base filtration, which may be further affected by the batch of chemicals. Enlarger light-source variations, which could also affect filtration for individual prints, can usually be eliminated with voltage stabilization.
If you're doing anything "creative" with the lighting or exposure, it's always a good idea to tell the printer not to adjust from their base filtration. Otherwise, they'll try to "fix" your "errors".