It takes far more energy to travel at 20 miles/h (Boosted) rather than 12 (E-Go).
Also range measurements can be deceptive. Perhaps the boosted board was measured in harsher conditions (more stops and starts say, or different weight, or different incline).
Boosted's battery has 99 Wh of capacity, while E-Go's has 230 Wh. Barring a significant difference in system efficiency, we would guess about a 2.3x difference in range under similar testing conditions.
Also range measurements can be deceptive. Perhaps the boosted board was measured in harsher conditions (more stops and starts say, or different weight, or different incline).