Why the bucket is filled faster in the downstairs tap than in upstair tap?

Assuming the taps are physically identical and the pipes leading to each tap are of a consistent size as well (and that the two are both fed directly from the same main with no intermediate Plumbing weirdness), the downstairs tap should have a (slightly) higher water pressure. Each foot of difference in height between the two taps will make a difference of a bit over two-fifths of a pound per square inch of water pressure.