Our minimum withdrawal for XRP is 1 XRP however you may receive a error dialog: "Coin amount must be greater than 20 XRP".

Because:
It's a reserve requirement from your XRP coin address to be activate on the network, so you may see that dialog if you withdrawal into your new personal address. 

Ripple (XRP) Ledger has a reserve requirement of 20 XRP, which is a minimum amount to store on your address. 20 XRP is used to activate your address and permit sending/receiving operations. It is an unspendable balance – you can’t withdraw it. To be able to manage XRP in Atomic Wallet, you have to deposit 20 XRP on your account. 

Why Ripple Ledger has this requirement?

Ripple ledger uses 20 XRP unspendable balance as a restriction measure to prevent the shared global ledger from growing excessively large as the result of spam or malicious usage. To submit transactions, an address must hold a minimum amount of XRP in the shared global ledger. You cannot send this XRP to other addresses. To fund a new address, you must send enough XRP to meet the reserve requirement.

The current minimum reserve requirement is 20 XRP. (This is the cost of an address that owns no other objects in the ledger.) Each new account must set aside this much XRP, which cannot be recovered or sent to others. You can read more about it here. This an XRP blockchain rule.

Why centralized services don’t require the deposit? 

Exchange services provide one paid address to many accounts. These users do not have their own unique address but only their own Destination Tag which assigns their accounts to that wallet. And hence, no activation needed from the user’s side. However, users have no control over the addresses. Atomic Wallet creates a unique new address and gives the user full control over it by providing access to private keys.
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You need to deposit 20 XRP on your address in Atomic Wallet. After that, you’ll be able to manage your XRP within the wallet.

Note: 20 XRP is an unspendable balance. You won’t be able to send it out.

(Source: atomicwallet)