Exactly. A research group will build an experimental setup and then do several experiments with it. In each paper you need to describe the experimental setup, so it makes sense to write that description once and use it in several papers while only changing the parts of the setup that you changed for the new experiment. Another case in which this often occurs is in the introduction. Each paper has a one or two paragraph introduction to the field. For instance papers about breast cancer maybe have an introduction that goes "Breast cancer occurs in X% of women over N years old, etc etc". Since a researcher's papers are most times in the same field, it makes sense to reuse those paragraphs in several papers. They may call this "self-plagiarism" but in my opinion there is nothing unethical about this.