One number versus a range, for the same height
For a 170cm man, the four ideal-weight formulas average to 65.9kg — a single figure. The healthy weight range for the same 170cm height spans 53.5kg to 72.3kg — an 18.8kg-wide band. These are two completely different kinds of answer to a similar-sounding question.
Where the ideal-weight point estimate actually falls
65.9kg does fall inside the 53.5-72.3kg healthy range, but not at its midpoint (which would be about 62.9kg) — it sits somewhat above center, closer to the upper half of the range than the lower half.
Why they were built for different purposes
The ideal-weight formulas (Devine, Robinson, Miller, Hamwi) were developed decades ago specifically for calculating medication doses, where a single reference number is operationally necessary. The healthy weight range comes from the WHO's BMI classification system, designed to describe a broad band of weights considered healthy, precisely because no single number can account for differences in muscle mass and frame size.
Which one to use for what
Neither is a personal target to hit exactly. The healthy weight range is the more practical everyday reference for most people, since it acknowledges natural variation. The ideal-weight formulas are more relevant in clinical contexts like dosing calculations, where they were originally designed to be used.