The heterogeneity of the variances over environments was a consequence of genotype-environmental interaction. If one assumes that interaction effects are random and unpredictable, then the variances over environments are a measure of stability. Basically, joint regression analysis views this heterogeneity of variances over environments to be a consequence of covariation between interaction effects and environmental effects. Both methods describe a data set. It is impossible to statistically show that one model provides a more correct description than the other.
Interpretation differs markedly depending upon the method chosen to analyze the data.
See Lin et al. Concepts and methods of analyzing regional trial data for cultivar and location selection. Heterogeneity of regression coefficients may reflect heterogeneity of interaction variances. Interpretation is more a function of the method of analysis than of the underlying biological interactions with environments.