Even if sizes and techniques were standardized, ingredients (and appliances!) weren't. The flour you got was the stuff the local mill made, the vegetables were

Even if sizes and techniques were standardized, ingredients (and appliances!) weren't. The flour you got was the stuff the local mill made, the vegetables were whatever variety was in season, and the meat was what you could afford or had left over. The older books relied on the cook to adapt them for the peculiarities of their kitchen to achieve a result they were already familiar with. Standardization only became a widespread thing in the early to mid 20th century. That's about when you start seeing cookbooks increasingly specifying precise quantities of known ingredients and sometimes tailoring to cooks who might not be familiar with the end result.