Moving Backward/Forward in Program Status

I saw this question on Kristen Carmean's blog post:

"True or false: You have three program statuses in your channel, in order: Member (Step 10), Filled Out Form (Step 20 - Success) and Engaged (Step 20 - Success). In this setup, a lead can move from Filled Out Form to Engaged but not vice versa."

The answer is marked as false.  Now, I always thought a lead could only move forwards in program status, so why does this example deem it acceptable for the lead to move backwards?

Thanks!  (And thanks to Kristen for putting together these fabulous resources!)