Multiple dice give you a discrete, "blocky" distribution similar to normal when they are added together. This occurs because there are more combinations that sum to results in the average range and fewer combinations as you approach the high and low end of the possible results. For example, with 2 6-sided dice, there is only one combination that produces 2, but there are 6 combinations that produce 7. In the case of percentile or d1000, since each die represents a different place value, even though they are combined to get the final result, the strategy retains the uniform distribution each individual die had (equal probability or flat line), as there are still an equal number of combinations (one) that produce each final result, as if you had rolled a single die.
With a uniform distribution, the chance for any particular result is the same for each possible result. Any particular result is one over the total possible results, so with a d20 for example, any particular result is 1/20 or 5%. Rolling a particular number or under is that number/total possibilities so rolling 15 or under on a d20 is 15/20 or 75%. The chances for rolling a particular number or over is the number plus the remaining possibilities, so rolling a 16 or higher on a d20 is 25% (since there are 5 possibilities, 16-20).
The chance of rolling 950 (exactly) on a d1000 (or 3 d10, where one die represents ones, one tens and one hundreds) is 1/1000 or 0.1%, the same as any other particular number. The chances of rolling 950 or under are 950/1000 or 95%. The chances of rolling higher than 950 are 50/1000 (951-1000 or 50/1000) or 5%.
The slightly odd bit about dpercentile or d100, d1000, et al, is that the highest die represents 1-10 (times the place), while the lower die or dice represent 0-9 (times the place). EDIT- Heh, that is NOT correct, at least not most of the time :) The 0 for the highest place ONLY represents 10 (times the place) when all other dice are 0, in order to make the result 1-100 instead of 0-99 or 1-1000 instead of 0-999, and so on.