Walking is the easiest most consistent weight loss method. The do Physical Activity Thing X times and metrics like BMI are aggregates for the population meant as generic advice (some doctors can't do math without a calculator sadly).
The gist of the advice is always to figure out your base metabolic rate and roughly how many calories you lose per steps/hours of walking. You can derive this experimentally for yourself by doing and eating the exact same thing for a few days in a row. You just need a good enough scale and a spreadsheet.
This drifts over time with your age and weight, so repeat the experiment occasionally. 5000-10,000 is the target range, maybe that's how they got 7k.
It doesn't matter if you hit your number daily or average it out weekly, as long as you get your steps in the calorie deficit occurs.