Hey Simon,
Andi gave you a pretty decent answer but let me bring a bit of clarification:
The Walk Forward Optimization changes the strategies. The complete backtest with the last parameters is different from the original strategy. Simply because it uses the last parameters. And it will take those last parameters only if they are better than the parameters in the original strategy.
The Walk Forward Validation does not change the strategy. If the strategy passes when in the Reactor, the same strategy will continue, with the same parameters. Here we are looking for over-optimization. If the Walk forward validation fails to create better segments by optimizing each one, it means that the original strategy might be over-optimized. Simply, the Walk Forward can not get a better result.
I hope that makes it clearer. If not, let me know.
Cheers,