It might be that your offer was used to supply the tiny remainder of some new trade position, which was mostly filled by using other lending offers? The engine combines available offers to allow a trader to open a specific position size.

Well -- intuitively I'd say, an amount of $0.0000000049 looks very much like a carry-over from some numerical imprecision accumulated in a previous calculation. (Such can typically happen when you subtract two numbers which are very close)