Some people, for whatever reason, have statistically unlikely luck. Either good or bad, but it's statistically unlikely. I've known people who can roll 16-18 using 3d6 almost every time with any dice handed to them. They can do so using dice towers, shaking the dice in their hand, using a cup, it doesn't matter. I've known people who rarely roll less then 17 on a D20, regardless of how it's rolled or what die they use. I've known people who almost always roll a 5 or less on a d20 too, regardless of how they roll or what die they use. Digital dice roller, dice tower, cup, rolled by hand, it doesn't matter. They can use any of their dice, any freshly bought die, or one handed to them by a 3rd party... it doesn't matter. They roll a 5 or less the majority of the time, with their high rolls being a 10. One guy has never once rolled anything higher then a 10, with 90% of his d20 rolls being a 5 or less. I know, we spent months tracking his dice rolls out of curiosity.
Some people will swear that you can "train" dice to roll high. I myself have noticed a trend where if anyone other then myself rolls one of my dice, it rolls low for the next 10 gaming sessions. Why? There's no logical reason, it just happens. I roll pretty average. I rarely roll nat 20's or natural 1's, but then again my dice rolls are pretty statistically average overall. Unless someone else uses my dice, in which case they roll low the next several sessions. Similarly, if I have to borrow someone else's dice... the borrowed dice tend to roll high for me. They then roll low for a few sessions for the owner. A statistician in my Pathfinder Society lodge is absolutely fascinated by this. He can't explain it, but it fascinates him.