Python - same GDAL command giving different results

Question

I've been using GDAL warp to re-project a tiff I have.

On the GDAL commandline I use gdalwarp -t_srs EPSG:3857 C:/test/test_file.tif C:/test/test_file_gdal.tif

Similarly, I wanted to run this function in python, so tried this

import os

command = 'gdalwarp -t_srs EPSG:3857 C:/test/test_file.tif C:/test/test_file_python.tif'

os.system(command)

However, despite them being the exact same command I'm getting 2 differently placed outputs. Why is this?