The "problem" is we see the obvious. Back up SS at 2B, SS goes down. The obvious move is 2B to SS and then figure out the best option for 2B. Which might be the LF. So LF to 2B and figure out best option for LF. Might be the back-up 1B sitting on the bench. And that's exactly what we'd do if making the changes live. But that's where the problem starts. I'd move my RF to LF, LF to 2B, 2B to SS and stick my back-up C in RF. You might say "WTF? That dude has 4 range!!!!"
So SIMMY does what we set up and then, if forced to make a move that we haven't set up, looks at specific defensive ratings to figure out what would be "best". C almost always end up at SS in that situation due to arm. Which tells me that the SIM favors arm over glove/range at SS. That might tell us something.