There are two factors at work here, both TigH alluded to:
1) Age: As you age, your body will change to some extent, especially as compared to how it was in your teens and twenties.
2) Body Composition: Weight is not a clear indicator of body composition. I've weighed 175 pounds three times in my life, but I can guarantee you that my body composition was different each time I weighed 175.
If you're losing weight primarily through calorie-restriction and reliance on an excessive amount of aerobic exercise, you could end up, once you get back to a previously-held weight, being "fatter" at that weight than you were the last time you held that weight -- by "fatter" I mean having a higher body fat percentage. By contrast, if you lose weight through calorie-restriction with a significant attention to getting adequate protein, and reliance on a combination of a moderate amount of aerobic exercise and at least a moderate amount of resistence training, you could end up "leaner" at that weight than you were the last time you held that weight -- by "leaner" I mean having a lower body fat percentage.