It's down to the VE (Volumetric Efficiency) of the individual engines, how well they can get gasses in and out.
The more restrictive, the higher the boost will be to make the same power as a car that breathes better.
Big valves, ported heads, high lift cams, large turbo's, all make for a better VE and so lower the boost reading.

You must remember that boost is relative to your engines VE and target BHP, you do not 'choose' your boost it is the result of what BHP you want and your engines spec.