First, I'm assuming the negatives are similarly processed - if they're developed in weak chemistry or for too short or too long that could obviously have an effect on the output. Also, with color film, temperature variation to a tenth of a degree fahrenheit affects the color of the film. Most good labs will produce similar results here though.
Scanning is an interpretation of the negative, just like when you print your b&w negatives with an enlarger. If you asked two different people to print the same negative, their results will differ based on their preferences. The same is true for scanning. If they're letting the scanning software auto-correct the exposure, saturation and color temperature, that also changes how the final image looks.