Just consider restricted media. Below a threshold, different things happen due to rates of metabolism, preferential uptake, etc. And this isn't to convince you

Just consider restricted media. Below a threshold, different things happen due to rates of metabolism, preferential uptake, etc. And this isn't to convince you that your water is "safe", but lead is excreted up to a threshold, and beyond that, it can begin to accumulate in bones and teeth, which is a rate. If yours is below the amount you're likely to consume over the given period that accumulation is likely to occur, then guidance says you should try to reduce it somehow to minimize the probability of long term health effects. I don't think either of you are at risk for acute toxicity from your water levels, so you could think of it like calories; you're fine with your current caloric intake, but should you double it? Diet lead, that classic taste.