In this blog of Section 5.2, the author said:
For the linear regression, we evaluate the overall model fit by looking at the variance explained by all the predictors. For the logistic regression, we cannot calculate a variance. However, we can define and evaluate the deviance instead.
Why the logistic model cannot calculate the "variance"? The dependent variable has already been logit transformed, and becomes a continuous variable from $-\infty$ to $\infty$. Why does it different from the "regular linear regression", as is written is Section 2:
Different from the regular linear regression, no residual is used in the model.