Why does it say "Letzame'ach"?
Malbim #1: The Kaved (Pi'el) form shows that it grew abnormally quickly. If not, why were the Pelishtim not careful to shave him regularly? They knew that his strength depended on his hair! 1
Since he said that he was a Nazir from the womb (verse 17), and this was interrupted, why should they expect his strength to return? And if it would return, would they not think that this will require giant locks, like before? Even six days growth is significant for Mitzvos, for the second shaving of a Metzora is six days after the first, and it is called Gilu'ach! However, also Radak implies that his hair grew long - refer to 16:22:2:1. (PF)
Why does it say that his hair started to grow like it was shaved?
Radak: Just like seven locks were shaved, so they started to grow.
Malbim: It started to grow immediately after it was shaved.
