Simply because 788L has been on other services like 159 (for
weeks on-and-off IIRC) and more after the Wrights went in. While
for 850P it only cameoed around once or twice after the Citaros
went in. So it's not the same case lah. 
Somehow I dun understand HGDEP logic... Now that HGDEP got many
Citaro intakes plus some more Citaros/K230UBs transferred from
SLBP, why can't they just do a one time MK3 replacement for bus
services like 43, 109 and 163 that still have 1-2 perm MK3s running
around?
Since the MK3s are going off very soon, might as well let all
the remaining HGDEP MK3s run SP for their final months. Rather than
letting the new buses run around wonderlessly without being
assigned anywhere...
Just my opinion, no flaming here.