yah this answers are from textbooks, anyway if mixture not uniform I would say dosage per tablet will not be desired dosage that you want,

Very good, this seems to be the answer Borek: and I want you to come to. However:

with this can it be explain like this:

if dosage per tablet is not acceptable(from what i think) it will result in every drug products will also having large deviation of weight between each other

and you're right back to your original point, which may be true, but doesn't follow logically from your conclusion.

Try to visualize -- consider a bowl full of black marbles -- 750 of them, and 250 white marbles. Consider the black marbles are excipients (you know the definition of that, correct?) and the white marbles are medicine. A person needs 25 white marbles for the medicine to work. So there are, potentially, 10 doses in the vat. So you pull out 10 separate piles of marbles. What do you get if the white and black marbles are evenly mixed? What do you get if the white marbles are all on one side?

So since non-uniform mix of powder affects weight and volume in powder,

Non uniform mixing affects the weight? Where do you get this statement? A pound of gold, and a pound of lead, melted together, are lighter or heavier depending on how well they're mixed? That's not true. Neither is it true for a medicine formulation. Likewise, volume can be affected by mixing -- but that's mixing of air into the powder, not mixing of powders.

we could said that purpose of uniform mixing is to get the desired concentration of the drug products

Now see, this is exactly right. But you didn't prove this, with your statements above.