Why were the Nimbus series Broomsticks the fastest?

Question

In Harry Potter canon, it was clearly stated at one point that Harry's Nimbus 2000 was the fastest racing broom on the market. Later, it was surpassed by the Nimbus 2001, which the Slytherin team used in Chamber of Secrets. Both the Nimbus 2000 and the Nimbus 2001 were faster than other broomsticks.

Now, my question: It was magic due to which an object fly. So, why were there speed differences between brooms, when presumably broomsticks were driven by the same magic?

Answer 1

In Quidditch Through the Ages says:

The charms placed on a medeival [broomstick] are similarly basic: It will only move forwards at one speed; it will go up, down, and stop. As Wizarding families in those days made their own brooms there was enormous variation in the speed, comfort, and handling of the transport available to them. By the twelfth century, however, wizards had learned to barter services, so that a skilled maker of brooms could exchange them for the potions his neighbour might make better than himself. Once broomsticks became more comfortable, they were flown for pleasure rather than merely used as a mean of getting from point A to point B.

QTTA - pages 2-3 - Evolution of the Flying Broomstick

Why were there speed differences when broomsticks were driven by magic?

Well, as the passage above notes, some witches or wizards were more skilled at making flying broomsticks as others. Just as wandmakers learn their craft through apprenticeship, perhaps broom-makers are the same and learn their craft over time. Brooms are able to fly due to charms placed on them; the skill and power of the witch or wizard imbuing the broomstick with its flying powers is going to vary. Some broom-makers will have stronger or more specialized magic than others, I'm guessing. Thus, you will see a difference between the different brands of brooms.

Regarding the Nimbus brooms, according to QTTA, the Nimbus Racing Broom Company:

galvanized the broom world in 1967. Nothing like the Nimbus 1000 had ever been seen before. Reaching speeds of up to a hundred miles per hour, capable of turning 360 degrees at a fixed point in mid-air, the Nimbus combined the reliability of the Oakshaft 79 with the easy handling of the best Cleansweeps.

QTTA - pages 50-52 - The Development of the Racing Broom

Nimbus has been at the top of the field since the introduction of the Nimbus 1000.

Answer 2

Some brooms are faster than others more or less for the same reason some wizards achieve better results than others when casting the same spells: they "have more" magic. This is already explained in @slytherincess answer. Three things to add:

1. Under the same logic of "brooms driven by the same magic should have the same speed" we could say that all wizards should be able to perform the same spells at the same stage of their education (or worse: same effects for the same spells, all the time). Does it sound reasonable? Not to me.
2. On the other hand, the performance of a broom cannot depend exclusively on the magic imprinted on it by the maker. Neville or Hermione, for example, would not fly really well even on a Firebolt, so I guess it depends on the rider as well. Remember that something similar happens with wands, where a wand will not necessarily convey the same magic for two different witches or wizards, with the consequence of different results in spell-casting.
3. You shouldn't forget the physics involved: aerodynamics, the density and lightness of the wood used, etc. This is also on the part of the maker. Maybe some of them have studied some basics of (muggle) physics and used this knowledge to improve their art.

Finally, note that a faster racing broom doesn't necessarily make a great quidditch broom, in the same way as a really fast car (i.e. a quarter-of-a-mile car) doesn't make a good racing car. In both cases, you need to make turns all the time, and this is not guaranteed if the broom is "just" fast. The Nimbus 2001 might have been the fastest-moving broomstick, but from the books you can tell that a Firebolt is far better for quidditch (Harry could "turn at the slightest touch", etc.). I guess both are really good overall, but there might be other brooms winning speeding contests at the wizarding X Games.