Understanding transaction fees with assets

[hybridd]

Transaction confirmed
#687e462b0f4500246fb3ba4191cd8c4914b65b27@474381
TRANSFER
From kuro113
To btstip-com
Amount 0.1 USD
Fee 0.1887 USD

Hi, so when transferring an asset from one account to another, here it paid my tx fee in USD (the asset),

where as here,

TRANSFER
From btstip-com
To hybr1d
Amount 0.1067 USD
Fee 42.42186 BTS

it paid the transaction fee in BTS. How do I know when the transaction fee will be paid in the form of an asset, or when it will just use regular BTS?

Thanks.

Further questions:
Is there a way to guarantee what the fee is paid in? This isn't evident from the api,

Code: [Select]

      /** Transfer an amount from one account to another.
       * @param from the name or id of the account sending the funds
       * @param to the name or id of the account receiving the funds
       * @param amount the amount to send (in nominal units -- to send half of a BTS, specify 0.5)
       * @param asset_symbol the symbol or id of the asset to send
       * @param memo a memo to attach to the transaction.  The memo will be encrypted in the
       *             transaction and readable for the receiver.  There is no length limit
       *             other than the limit imposed by maximum transaction size, but transaction
       *             increase with transaction size
       * @param broadcast true to broadcast the transaction on the network
       * @returns the signed transaction transferring funds
       */
      signed_transaction transfer(string from,
                                  string to,
                                  string amount,
                                  string asset_symbol,
                                  string memo,
                                  bool broadcast = false);