When you try the following python script with the ximagesink or xvimagesink as the sink element in the pipeline. the output is different in the cropping of the image.

import pygst
pygst.require ("0.10")
import gst
import time

pipeline = gst.Pipeline ("pipeline")
source = gst.element_factory_make ("videotestsrc")
videocrop = gst.element_factory_make ("videocrop")
sink = gst.element_factory_make ("ximagesink")
#sink = gst.element_factory_make ("xvimagesink")

pipeline.add (source)
pipeline.add (videocrop)
pipeline.add (sink)

source.link (videocrop)
videocrop.link (sink)

pipeline.set_state (gst.STATE_PLAYING)
time.sleep (2)
videocrop.set_property ("bottom", 100)
time.sleep (2)