For me, different spicing. When I prepare these for myself I use different spicing, and find that the results satisfactorily emulate the different flavour of each that I expect to experience when eating in a local (Kent/S E London) BIR. And at a superficial level, I expect tandoori chicken to be a full saturated carmine/crimson whilst I expect chicken tikka to be far less saturated and more towards the yellow end of the spectrum.
** Phil.