Leighton is from the Old English lēah-tūn — meadow town, settlement at the clearing. Leighton Buzzard — English town in Bedfordshire. The name's modern surge as a girl's first name is directly traceable to Meester's Gossip Girl fame.
Leighton does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
In Pythagorean numerology the letters of Leighton reduce to 9, The Giver. This is a traditional interpretive system, not a factual claim about the name.