Converting from BTC to XRP is rounding to 4 decimal places after the quote, if the result is more than 0.01, and if the amount is less than 0.01 its doing it to 17 decimal places.
Right now (at a price of 0.00001387XRP/BTC), 720 XRP gives quote of 0.00998727 BTC but then if you accept the quote it's actually performing the transaction as 0.00998726764375112 BTC => 720 XRP.
But 721 XRP is quoted as 0.01000114 BTC and then if you proceed with the transaction you get 0.0100 => 721 XRP.
You get 0.01 for 721 through to 724, but if you try 725 it rounds it up to a cost of 0.101 even though the actual quote was for 0.01005662.
The client never used to do this rounding. The amount you were quoted was the amount that was given during the transaction.