...
Seriously?
No.
Short humans and elves and orks don't get lumped into the dwarf metatype. Tall humans, elves and orks don't get lumped into the troll metatype. Dwarfs are a distinct metatype with distinct benefits. Just because someone plays an ork with dwarfism (the condition that afflicts people nowadays, not the UGE kind), doesn't make them a dwarf; they'e just a short ork. Just because some folks are freakishly tall, doesn't mean they have dermal deposits and horns )or the over-long arms) to make them trolls. Orks, elves and humans don't have thermographic vision, which dwarfs and trolls do.
To argue all shorter than average people are dwarfs and all taller than average people are trolls in the canon Shadowrun is flat-out idiotic. If it is something you do at your table, say as much.
Back to the point... Maybe Seattle itself is short on (ha, get it?) the diminutive aspect of metahumanity. They had the Night of Rage, the falling out with the Ork Underground, there easily could have been an exodus. It also isn't impossible for the city to have a higher-than-average human population compared to the world-wide average. Not to mention those percentages you're talking about only cover the SINners, which leaves plenty of undocumented orks and trolls to bring the percentages back in line.