Strengths are stable under small changes in the model.
The two models have words defined by different labelers
This model allows extra freedom: different tones are allowed to define their targets differently
This model allows less freedom: all tones have the same type of target.
In fact, we fit 15, slightly different models to the data, and all the pairs look more or less like this.  The red curve shows that for most syllables, there is a simple relationship between the strengths derived from the two different models.