  Recent studies of lensing clusters reveal that it might be fairly common for
a galaxy cluster that the X-ray center has an obvious offset from its
gravitational center which is measured by strong lensing. We argue that if
these offsets exist, then X-rays and lensing are indeed measuring different
regions of a cluster, and may thus naturally result in a discrepancy in the
measured gravitational masses by the two different methods. Here we investigate
theoretically the dynamical effects of such lensing-X-ray offsets, and compare
with observational data. We find that for typical values, the offset alone can
give rise to a factor of two difference between the lensing and X-ray
determined masses for the core regions of a cluster, suggesting that such
"offset effect" may play an important role and should not be ignored in our
dynamical measurements of clusters.
