When you roll a perfect die with 6 faces, each face has a probability of 1/6 to be on top. However, there is no perfect die, and the probabilities will change a tiny bit around this 1/6.
A simple thing you might check before predict the output is to have a close look at the dots on the die i.e. if the dots are increasing or decreasing the mass of the die.
If the dots increase the mass of the die then there is a shift on its center of gravity (CoG) to the zone with more mass. Thus, face 1 is lighter than face 2, face 2 is lighter than face 3 ... and the heaviest is face 6.
If you play with a die where the faces 1-2-3 are opposing the faces 6-5-4, respectively. Then, the Cog is closest to the faces 6-5-4. As the heaviest has a greater probability of end up on the bottom at the end of the rolling, then, there is a higher probability to get small numbers than bigger numbers.
So, next time you play a CATAN game with your friends, you can customize the dice, you can use aluminum to build the cubes and iron to build the dots, then always pick the smallest numbers... Easy win!
Have a good weekend! See you soon.