There's not one single standard value or exchange rate for any particular currency to Bitcoin across the world since there are different markets / exchanges. Your app could be basing off just one of the many exchanges out there.

Anyways for Bitcoin wallet apps themselves, I don't think it's safe to exclusively pool your Bitcoins within a single wallet that can be accessed through your phone. It's convenient, but I suggest you diversify a bit with other wallets that can only be accessed through browsers + two-step email authentication.