This is a very common misconception.
Part of the reason is that twins are far more memorable than singletons. This is specially true if the birth of the twins has happened after many years of infertility , because of IVF.
On the other hand, singleton IVF pregnancies are not very memorable - and infertile couples will often not even share the information that they did IVF to get pregnant.
Also, multiple pregnancies after IVF used to be much commoner in the past , because many IVF clinics would transfer multiple embryos at one time. They didn't have very good success rates, and in order to increase the chances of success, they would put many embryos in the hope that at least one would stick ! The problem was that if multiple stuck, patients would end up with multiple pregnancies.
This is an outdated practice today, because the complication rate with a twin pregnancy is higher. While many infertile couples are excited about the possibility of getting twins , because they think of this as an " instant family in one cycle", and while it's true that a twin pregnancy is better than not getting pregnant at all, the truth is that this is not in your baby's best interests.
Global best practices recommend that you should transfer only a single blastocyst at one time , and freeze the rest. This way , the cumulative pregnancy rate is higher, and it's much safer for everyone !