Since the answer came in many parts, and from many people (and I kinda messed up with the question when I originally posed the question), it's a bit hard to get all the bits scattered around in the various answers and comments.. Therefore, here is:
The Official Spoiler
The answer in a single word, is
Here are all the intended clues, piece by piece:
Dan Hedfelt is a small gaming equipment manufacturer.
"Dan Hedfelt" is, of course, an anagram of "left handed"
Recently, he upgraded the machine that makes six sided dice, but the new machine started acting up, and he faced a production quality issue: the machine worked otherwise perfectly, but for about once in every ten dice, it somehow swapped every side's print to the exact opposite side.
This is the geometry bit. Swapping two opposite sides of a dice changes its chirality, meaning that that the dice will become a mirror image of itself. Swapping all the sides is equal to three such swaps. Any two swaps cancel each other, so the result is a mirror image of the regular dice.
Apart from geometry, the 1-in-10 refers to the approximate ratio of left-handed people in the human population.
Dan was initially worried about this, but since the error left the resulting dice perfectly fair and usable, he just handed them out as small gifts for his customers;
Reading only the verbs in this sentence, you get "was" "left" "handed". (Yeah, I wasn't surprised nobody got this..)
he was actually feeling a kind of affinity with those misprinted dice.
The nature of this affinity is the crux of the matter, and will be resolved shortly..
After signing the contract to sell the old machine, he then headed to the tennis club, since his twin brother had once again challenged him to a rematch. Once there, his brother noticed a smudge on Dan's hand. "Oh yeah, that happens a lot", Dan replied. He then promptly proceeded to defeat his brother, as was usual.
These are all hints about Dan's left-handedness.
- Statistically, it's about twice as likely that a person is left-handed, if he has a twin.
- Left handed people writing with a pen do often get smudges on the hard-to-see part of their hand, since when writing from left to right, the left hand tends to touch the paper at the very spot where the wet ink is. Apparently this happened when he was signing the contract.
- When practicing tennis, the vast majority of your opponents will be right handed. (This is true, even though lefties are somewhat overrepresented in tennis: 15 of the ATP top 100 are southpaws.) Therefore, when playing against his twin (who is right-handed; twins often prefer different hands), Dan has the advantage of having practised more against similar opponents.
So finally, here’s the question: Dan was somehow able to tell the misprinted dice from the regular ones. How?
The geometric term "chirality" actually is just a synonym for "handedness". The regular western dice are said to be "right-handed", meaning that when you look at the 1-2-3 corner, the numbers run in the counterclockwise direction. The misprinted dice, having their chirality reversed an odd number of times, are "left-handed", meaning that the numbers 1-2-3 run in the clockwise direction.
So, the reason for the affinity was that
Dan was left-handed, and so were the misprinted dice.
Shouts out to Kamome, who solved the original puzzle without needing most of the clues and got the tick for his efforts, RnRoger, who figured out the majority of the remaining clues, Foo Barrigno who finally connected all the clues to the title, solving the newly edited question, and M Oehm, who was the first to figure out the anagram.
Showering each of them with upvotes wouldn't be a bad idea at all.
Thank you for your interest!