Or it could be a compilation of other things besides the 15 extra keys. Like...

#1: Lack of a 76 key version. (already mentioned by Bachus)
#2: NO real sampler.
#3: FSX keybed still too light = toyish.
#4: Too expensive $$$$$ for what you get.
#5: Still can't edit Voices microscopically.
#6: $5,000+ yet NO - AWM"2" Tone Generation.
#7: Internal memory still miserly at just 6MB.
#8: NO "balanced" Input/Output connections
#9: NO XLR/Stereo Mic input
#10: NO Line IN for recording (that I'm aware of)
#11: No MIDI "Thru"
#12: Still only 128 note Polyphony... @ $5,000 you'd expect more e.g. 256.
#13: Plastic shell casing = toyish.
#14: Wasted space. Example... cup holder on right.
#15: It's a Yamaha???

All the best,
Mike