the issue with low star value maps is that you have a highly varied selection of dificulty settings, with almost jo standardized AR, OD being pretty varied, and CS ranging from huge to the standard 4, not to mention because these maps are supposed to develope the most basic game mechanics, primarily understanding how approach circles work, reaction time, and BASIC rhythm sense they are very lenient with ranking standards, which leads to some weirdly timed notes, semi random patterns on some maps, and most songs being vastly under mapped. Since the ranking standards aren't as strict on easier maps this also means just about anyone can map one of these and get it pushed through wether you are an expert mapper or just starting out. All of these things result in a very unstable level of difficulty in the 1-3 star range.
outside of that star rating is fairly unreliable, it's more of a rule of thumb than a science, you have a lot of very subjective factors that determine how hard a beatmap is from reading, BPM, note density, spacing, streaming, wether or not there are a lot of weird stacks(ie: doubles, 4 stacks) and most of these factors don't exactly have a metric to measure difficulty by, so it's typically more like a rough guess than a concrete rating