Same here, Standard 243bhp - rolling road came out 254bhp with de-cat. RB5 nearly 10 years old.
I think its down to how efficient it is mechanically. i.e. a car thats had a hard life, up hills, constant heavy driving will be more worn generally and can't run as well. Depends on usage rather than age i think
I thought about this on holiday in Devon going up the hills, Porlock Hill, in particular. It performed amazingly but I did think it must put tremendous pressure on gearbox, diffs, driveshaft etc.
Regards, Joe