Being upstairs doesn't prevent a path to earth, it merely gives it a higher impedance/resistance - bear in mind your body resistance might be a couple of hundred K, so it forms a potential divider with the resistance to earth.
Yeah, I just thought the resistance would be so incredibly high (many Giga ohms?) that you wouldn't feel anything at all (1850v source, according to the meter, which is what i'm testing really). Sitting on this office chair lessens the sensation. I thought the chair alone with all it's plastic would have a massive resistance too.