Good to hear.
This is all expected. The buttons are actually completely independant of the scroll frame and vice versa. They just happen to be anchored to it. For all the scroll frame knows, you could be simply printing different numbers in the update function instead of doing something with the buttons.
You have decided that 8 buttons be created (as per NUM_BUTTONS), and the size of the scroll frame is smaller than the total height of these. That's all there is to it. Again, the scroll frame is completely independent of the buttons, and has no idea how much screen space they take up. If you're using constants for the number of buttons and their height a simple solution would be:
Code:
scrollFrame:SetHeight(NUM_BUTTONS * BUTTON_HEIGHT)
which I just noticed you were nearly already doing, you just used 6 instead of 8 which NUM_BUTTONS was set to. This way the appropriate height will always be calculated when you change the amount and height of the buttons via the constants. At least, as long as the offset between each button is 0.
The scroll bar will always remain in its original state (visible and inactive) until you first call FauxScrollFrame_Update, which you do in the update function. The reason it disappears when you do so is that you have enough buttons that the entire list can be displayed without scrolling (you have 8 buttons and 8 items in the list). If you add one more item or remove one button, you will see that it works!