I would guess it's because though Boston has some (relatively) lower density areas than Somerville which drag its overall average down, places like Back Bay are denser than anywhere in Somerville due to the higher density of higher-rise housing. Somerville doesn't really have any neighborhood that is entirely 4-5 story brownstones plus high rise condo buildings. There are a lot more three-deckers in Somerville, which don't house quite as many people.

Somerville is the most dense overall, but Boston has a lot of low density neighborhoods like Brighton and West Roxbury that bring it's average down. Parts of Back Bay and South End are more dense than anything in Somerville.

You don't have to build vertically to get density. But if you want the densest single 1/4 square mile in an entire state it's probably going to involve building up.

> Somerville considering it's in the top 10 most dense cities in the US

I would believe that Somerville is the most dense from a "number of residential properties per unit area" standpoint. But when each of those properties contain only 3 to 5 people living in them each, the actual population density isn't going to be crazy high. Compare Somerville to places that have a lot of multistory condominiums (with hundreds of people living in them each) from a "number of people per unit area" measurement... and the Condo's will blow Somerville out of the water from a population density standpoint