I believe Lip has it correct in it being secondary growth.

This is just my twisted method going here, but here's my take.
I don't know enough about growing plants with your light source so it's hard to tell how fast they will develop. That said... I'd keep vegging for at least 2 more weeks. Then go to 12/12

Your plants look healthy and thriving...

Good Luck which ever of the many paths you have to decide to take...