The way that it works is that CR only uses the results of a model if they have enough survey returns for a given model. This applies to all manufacturer's models. In the case of Mercedes, they only received enough survey returns for two Mercedes models. What this tells me is that either most Mercedes owners do not subscribe to CR, or, most Mercedes owners that do subscribe do not take the time and trouble to fill out a survey.
CR does not specify which models they are basing their rating on. My guess is that, in the case of Mercedes, they are likely the two cheapest models, which also happens to be the two least reliable and most trouble-prone Mercedes models. If the E-Class were included the results would likely be different. In the case of BMW, the cheapest models tend to be just as reliable, and maybe MORE reliable (which has traditionally been the case), than the higher-end models.
I suspect you have a valid point. MB top 2021 sellers were, in order: GLE, C-class, GLC, GLB The less expensive models are Mercedes least reliable. Not sure about the GLE.