Hadley sits next to 4 phonetic neighbors — Bradley, Radley, and Hartley are the closest matches by sound. Most share Hadley's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"from the heathland"
"God has shown favor"
"God is gracious (through John)"
"God has uplifted"
"placeholder birth name turned given name"
"wood, timber"
"of the forest men, from Norman Freselier"
"bold, brave"
"spiral, coil"
"little lord, champion"
"My God is Yahweh"
"age, era, vast expanse of time"
"his peace"
"from Hadria (a Roman city)"
"warrior"
"palm tree, upright"
"diminutive of Jakob"
"king"
"a Breton, from Brittany"
"modern coinage; possibly town settlement"
Similar sound: These names share phonetic patterns with Hadley — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old English origins, giving them the same cultural heritage as Hadley.
Rhythm: Hadley is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.