Hi Susana and welcome to the forum,
You're confusion is something we see from many new members, so you're certainly not alone. I think this link will give you a much better understanding of how the market actually works and will explain what might be happening in this circumstance.
One really important factor to always consider with a Dropshipping supply model is that while Dropshipping offers some great advantages, profit margin is rarely ever one of them. The only way to have a good Dropshipping margin is usually buying direct from an actual manufacturer of a product because then you only have two people in the chain.
Most Dropshipping companies are NOT manufacturers, they are essentially a third party that buys in the stock from manufactures in bulk and then Dropships for their customers, so now you have 3 people in the chain and that of course means much less money to go around. From there it only gets worse, because you can have situations where another party will then buy in smaller volume from the actual Dropshgipper and then on-sell to their customers, meaning that you are now 4th in the chain and all the potential margin as evaporated along the way before you even get a chance.
So Dropsihipping can be a very intertwined network of parties and is why researching suppliers is of the utmost importance to ensure you are as far up in the food chain as you can possibly be. That said, as the post points out in the link above, Dropshipping will only ever give you a chance of competing against other Dropshippers, it will rarely, if ever, give you a chance of competing effectively against a seller using a Wholesale supply model.