Misleading flight bookings

I booked a flight and the funds were deducted and flight booked but a day later I was told the booking is cancelled because the flight was fully booked. And now they tell me to book another ticket which now the difference was already going on R1700,00 and I am supposed to pay the difference. My question is how does the website allow me to book a fully booked flight and why am I paying a difference for their glitches.