[Blackmax] Physics question

[Silje Hattrem Raddum]

Hi BlackMax authors,

I've generated some events with the two-particle final-state option, and 
I've noticed that the branching ratios for leptonic and (semi-)bosonic 
decay modes decreases as the Planck mass (BH threshold) is increased. 
Some examples:

M_P = 1.5 TeV
Br (BH -> ee) = 0.025%, Br (BH -> gamma + gamma) = 0.013%, Br (BH -> Z + 
X) = 3.14%

M_P = 2.0 TeV
Br (BH -> ee) = 0.018%, Br (BH -> gamma + gamma) = 0.009%, Br (BH -> Z + 
X) = 2.41%

So, my question is: can you give me a simple explanation for this effect? :)

Cheers,
Silje Raddum, University of Oslo