The multiplier value on any given orb is generated randomly by the slot system. During base play, these multiplier symbols only apply to the cascades of the specific spin they appear in. Once the bonus starts, however, the mathematics change entirely.
The total multiplier becomes persistent. If a 10x multiplier lands on a winning free spin, it adds to the global meter. If a subsequent spin hits a win alongside a 50x multiplier, the game references the full accumulated total multiplier to calculate the payout. This is the mechanic that enables the 15,000x maximum: multiple orbs stacking over successive tumbles within a single bonus round.
Here is a small example to make this concrete. Imagine you enter the free spins round and your first winning cascade includes a 5x orb. Your total multiplier is now 5x. Two spins later, another win lands with a 25x and a 100x orb. Your total multiplier jumps to 130x. Every subsequent win in that round gets multiplied by 130x, and the number keeps climbing. That snowball effect is what makes the free spins round so volatile, and so potentially rewarding.