Flipping method to translate XYZ to TMS right?

[M.G. Ting]

node-mbtiles/lib/mbtiles.js

Line 159 in aaf9290

 y = (1 << z) - 1 - y; 

Why it is 1 << z but not 1 << (z - 1)? Cause i cannot load right tile using a mbtiles file connection.