I've recently read a paper which used generalised linear mixed models to estimate mean annual and monthly values for the response variable in the model . The response variable was a normally distributed continuous variable and year and month were used as predictors in the model. The annual and monthly mean were then calculated for the response variable. How is the annual and monthly mean estimated for the response variable in a generalised linear mixed model? How would the mean calculated by a generalised linear model differ to a mean calculated by hand?