I have always been taught to pronounce the letter "zaa" as "zaa" never as "zay" or "zayn". Are these all correct forms of how to pronounce this letter?

Question
bismihi taalaa
assalaamualaykum warahamtullhi wabarakatuh

I have always been taught to pronounce the letter "zaa" as "zaa" never as "zay" or "zayn". Are these all correct forms of how to pronounce this letter?  Why is there a difference of opinion?

jazakumullah kheyr

Answer
Wa alaikum assalaam wa rahmatullahi wa barakatuh,
The letter is always pronounced with a "z" sound when reading the Qur'an, but there are different names for the letter, the most common name being "zay", but it is also called "zayn" in some areas of the world.  That there are different names for one letter should not bother us.  The letter is still pronounced the same though, and that is the important matter for us. 
In English the letter "Z" is called Zee in North America and called Zed in Europe, but again, the letter is pronouced the same. 
Wa lakum -ul-Jazaa'.
Wa assalaam alaikum.