How come the citroen C2 VTS has 15bhp more than the VTR when they both share the same engine

it usually come from more compression

different electronic engine management systems

Engine Tuning.
This can be done on most cars of different specs but with the same engines.

power to weight ratio!

whats ''mclaren'' on about? same car, same engine!
the power to weight ratio is gonna be the same.

proberly different management, reprofiled cams & less restrictions on the inlet manifold.

a prime example is fords zetec-e engine:

a 105bhp 1.8 can be upgraded to a 130bhp 1.8 zetec by swapping the cams, ECU & inlet manifold.

strange innit?

i guess its done for the purpose of insurance, i.e the vtr being cheaper & 2 groups less at group 6, personally i'd go for the vts.