10-15 is pretty extreme, no? Also need to consider what they are dropping from and their ages/sex.
Either way, good for you. Even if its a slow go you are doing something positive for yourself and again, thats an approach the majority of the population wont have the discipline to attempt, let alone make a habit of.
But even then, most of that weight loss (or a good chunk of it) is going to be water.
I get the obsession with the scale but it's probably the most destructive measurement a person can do on a health journey.
There are so many variables that can f*** with your numbers on a day to day basis that it really isn't worth tracking any more than every month (if not even longer spurts) and depending on what you're doing, it may not be worth tracking at all.
If you feel better and you're showing some signs of improvement (if you're lifting weights, you're able to lift more weights or do more reps, if you're running you're able to run further/with out fatigue, etc) those are the things that really matter. Any type of body transformation will come along with all of that provided that you're doing the right things in the kitchen (it's very, very, very hard to out work a bad diet.)
Taking the actual time to do physical measurements (Waist, chest, etc) is a much better exercise over time. The scale may only say someone is down 4-5 lbs but if you've lost a bunch of inches and you're able to comfortably fit into clothes that you haven't been able to wear for a while amd you look/feel better then who cares what the scale says?