Jeff sits next to 3 phonetic neighbors — Geoff, Jeb, and Jesse are the closest matches by sound. Most share Jeff's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"divine peace, God's peace"
"beautiful fragrance"
"supplanter (through Jay from Jason/James)"
"divine peace (God + peace)"
"from Lucania, southern Italy"
"sad, sorrowful"
"visible mass of water droplets in sky"
"lean, slender warrior"
"my vision, my sight"
"of the forest, woodland dweller"
"supplanter"
"son of the ace, son of excellence"
"variant of Mason (stone worker)"
"strong will, brave resolve"
"faithfulness, devotion, steadfast allegiance"
"crown, wreath"
"moon, of the moon"
"good valley"
"small dwelling, storage chest"
"father holds"
"noble battle maiden (via Audrey)"
Similar sound: These names share phonetic patterns with Jeff — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Germanic origins, giving them the same cultural heritage as Jeff.
Rhythm: Jeff is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.