I found something similar, with a very simple example.

On 64-bit linux, python 2.7.2, numpy development version:

In [22]: a = 4000*np.ones((1024,1024),dtype=np.float32)

In [23]: a.mean()
Out[23]: 4034.16357421875

In [24]: np.version.full_version
Out[24]: '2.0.0.dev-55472ca'


But, a Windows XP machine running python 2.7.2 with numpy 1.6.1 gives:
>>>a = np.ones((1024,1024),dtype=np.float32)
>>>a.mean()
4000.0
>>>np.version.full_version
'1.6.1'