When I first read 390bhp and saw the mods I thought no way, more like 350, but the OP could have been a victim of a crappy RR.
The 1st RR I did was my stock 335i at the BMW show and it came out at 345bhp (about 15% over). I went away believing this was a right result until I realised there was no way this was possible.
I was standing with jimmylimmy (member on here) who had just had only a remap done on his 335d and his was reading IIRC around 385bhp.
I have since had it re-done at Evolve and it came out a 370bhp. I expected about 385 as it's JB4 + DCI + Performance exhaust. (This would have been 425 on the BMW show roller
)
So, I guess it could be a genuine mistake by the OP.