Why would a surname be edited and change like this?

[stevez]

Attached is a 1771 birth record for Giovanni Maria Pianalto, born in Recoaro Terme, Vicenza. His descendants are Pianalto but his father Giovanni's surname is Brunialti (underlined) with Pianalto written above it in a different color. On the 1731 marriage record of his father Giovanni, his surname is Brunialti, so it appears that his son Gio Maria's family name changed to Pianalto. What could be a reason for this? Brunialti and Pianalto are very common surnames in Recoaro.