When you see an offer with a chance to double or triple your purchased amount, it simply means that the lucky winners will receive additional chips, as shown below.
Here's how it works in detail:

In this case above, if you purchase e.g. 1 000 000 000 chips and win the raffle, we will add an additional 2 000 000 000 chips. This makes the initial purchase tripled (3 000 000 000).

Here, if you make tagged purchases for e.g. 2 000 000 000 chips and win, we will double that and add another 2 billion to your balance. This will make 4 000 000 000 chips in the end.