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.