What causes stock to turn out clear and clean tasting sometimes but cloudy and heavier other times using the same low simmer method

I regularly make chicken or beef stock at home and my process is very consistent. I start with cold water, bring it up slowly, skim carefully, and keep it at a gentle simmer without boiling for about two to three hours. Despite this, the results are not always the same. Sometimes the stock is very clear with a clean and defined flavor, and other times it turns slightly cloudy with a heavier mouthfeel and a duller taste. I am not adding salt or acidic ingredients and I am not stirring aggressively, only occasionally moving the bones to prevent sticking. I am trying to understand from a technical standpoint what variables most commonly affect clarity and texture in this situation. Is it protein release timing, temperature control, bone preparation, or skimming technique. I am looking to understand the mechanism rather than rely on trial and error. submitted by /u/No-Coyote2836