In spanish (latin language, like italian), we have the rule that says: "n" changes to "m" before "b" and "p".
Giambattista,
Giampiero and
Giampaolo follow this rule; however, these names should be "gianbatista, gianpaolo, etc." (because of Gian), but they turn into Giam... because of that rule.
Hope it helps.