TIMBER IS A NATURAL PRODUCT: each piece is unique in colour and grain, no two pieces are identical, even if the timber has been stained , (even black ) each piece will have a different depth of colour as the undertone of the natural timber reflects through, again this applies to the grain of the timber, if the boards are a sliced veneer, it can be expected that the colour will be more even due to the thin slices of the top timber, if the boards have been sawn one can expect more variation in colour and grain and grading of the timber.
Oak has a more consistent colour when compared to the Australian Spotted Gum or Ironbark, once installed you will notice all types of grains, such as tiger marks( swirls) in the timber, other boards will have large patterns of grain, others may not even show any grain and appear very consistent.
This is what makes timber unique, again all timber ages and oxides, this process is a natural process and darkens the timber when the timber has been exposed to the environment such as air/light, oxidation is an ongoing process and cannot be stopped, even smoked or stained product will oxidise and change colour. (see how to care for your floors-maintenance.
If you as a consumer are concerned about colour /grain variation then you need to call your supplier immediately to identify your concerns.
Batches and locations also affect the overall appearance of timber, such as Spotted Gum (Queensland) which is much darker than Spotted Gum from New South Wales, Oak from Russian, has a much more consistent grain when compared to Oak from France/Belgium or China.