Unusual Wheel and Layout of the Numbers.

Numbers skip by 3 digits on the Wheel - 1 ,4, 7, 10 etc and then after the Zero start again 2, 5, 8, 11 etc. As you noticed the frets (buckets) are round and the ball fits perfectly in to them.
Glad you liked the post.
Take care,
"Mr. Roulette"