I don't understand why it be sent using a random number though, I checked my other transactions and they also were sent using a random number,
That's just how most service works. The bitcoins in your deposit address will be sent to their cold storage (because cold storage are safer), leaving it with zero balance. An address with zero balance can't send bitcoins so they use their hot wallet instead.
Sending the bitcoins back to your deposit address before sending them to your withdrawal destination would cost them the transaction fee (plus it may increase the confirmation time).
so where has the refund gone ?
It's gone to paxful's treasury.