Concrete is not a homogenous, uniform material. Just like natural wood or stone, the finished concrete’s appearance can have subtle aesthetic variations of shade, colour or pattern, depending on the materials used, the casting and curing process and the processing and finishing techniques.
Concrete is a natural, handmade material. Many factors such as humidity, sand lot and cement lot can affect the final appearance of the concrete. Even if colour formulas record the type of sand, type of cement, and exact measurements of pigments to 1/10th of a gram, colours can still vary because of the aforementioned factors.
Variation, especially colour and visual texture, almost always occur to some degree or another. The greatest degree of variation is often seen between old samples and newly finished concrete, because the ingredients used were not identical. However, if carefully executed, all of the concrete within a particular job should look the same, even if the countertops looked subtly different from the sample. In other words, all of the variations will be similar so that the finished product looks alike.
Another factor that contributes to perceived variation is simply the size of samples. A4 size sample, or even a A3 size sample, will look different than a whole countertop simply because the small sample does not capture all of the natural, random variations that will occur over a large surface. The colour in a A4 size sample may appear completely uniform, but a whole countertop in that exact same colour will not appear as uniform. The same situation occurs with other natural materials such as granite. If your client prefers perfect uniformity, solid surface might be a better countertop option.