An abacus stores a number the same way written decimal does: with place values. Each rod is one place, from units on the right to billions on the left.
Place value on the sorobanA bead only counts when it is pushed against the central bar. On every rod one heaven bead sits above the bar and is worth 5; four earth beads below it are each worth 1. Reading a rod means adding 5 if the heaven bead is at the bar, plus the number of earth beads also touching the bar.
Because of place value, a single bead can represent 1, 10, 100 or billions depending on its rod. Pushing the 1,000s earth bead changes the whole number by 1,000, not by 1.