A Bear Growl at the Dog Park

Misha and Missy see each other from across the park; they run towards one another. It was a nice moment. More and more dogs arrive at the park. Misha greets all of them - the humans, not so much. A couple walks in and pets her - I'm surprised but proud.
But…..a few minutes later, Missy is behind Misha, that same couple walks towards her and there it is: that deep bear growl followed by a protective, deep, bark. Misha approaches them and smells. Everything is ok. But why did she do it? The only thing I can think of is that she was protecting Missy. She's never done that before, and the oddest thing is that she had already allowed that couple to pet her. I stood up immediately and corrected her. She sat on command and waited.

Another couple entered the park and then this happened:

This random person comes over and pets her, massages her, she lays down on the ground for him and asks for belly rubs. I praise her immediately after the interaction is over. 

The previous event followed by something like this is what's confusing to me (both events happened within 10 minutes of each other. The person in the picture is someone random, not related to the couple she growled at). If she didn't like people all the time, I would understand. But her feelings towards people change constantly. Is it different energies she's feeling? I am not sure.

This person went up to her multiple times and it's as if she had known him for years. 

Well, Misha, you never seize to surprise me and confuse me. Let's continue today's training. 

Update soon.