python - pyProj Coordinate Transformation Incorrect

I'm using pyProj to transform extents between two different geographical systems. In a case when both my systems are the same I expect to get back the same extents as inputted but this isn't happening for the below mentioned case:

import pyproj
inProj = pyproj.Proj("+init=EPSG:{0}".format(3857)) # Mercator
outProj = pyproj.Proj("+init=EPSG:{0}".format(3857))
x =  (-20037507.0672, 20037509.6184)
y =  (-1467048.29156, 8625918.8737)
print pyproj.transform(inProj,outProj,x,y)

The output comes out to be : ((-20037507.0672, -20037507.067178484), (-1467048.2915600014, 8625918.873699998))

Which is incorrect as my X-Coordinate extents are same for xMax and xMin.

If I change my co-ordinate systems

import pyproj
inProj = pyproj.Proj("+init=EPSG:{0}".format(3857)) # Mercator
outProj = pyproj.Proj("+init=EPSG:{0}".format(4326)) # WGS84
x =  (-20037507.0672, 20037509.6184)
y =  (-1467048.29156, 8625918.8737)
print pyproj.transform(inProj,outProj,x,y)

My X-coordinate output comes out to be : (-179.99998854118687, -179.99998854099357)

Which is again incorrect. Is this due to a domain issue in pyProj or is there any other reason ?