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I'm going to throw my bit in here ...
It is very rare that the exact number of stars are quoted for an open cluster or globular cluster. I've seen numbers quoted for clusters varying in the hundreds or even thousands. Often it is hard to determine whether a star actually belongs to a cluster or the measurements have simply just not been done. You only have to look at the variations given for the size of a cluster.
The centre of a cluster is purely an approximation based on one source's reading of the data and/or visual assessment off an image.
With our current level of data, we cannot accurately give the centre of a cluster to the nearest second of arc when it may be disputed to the nearest minute of arc.
This not only applies to clusters but also to larger galaxies or planetary nebulae, where the outer boundaries are just approximations.
Don't be fooled into thinking these numbers are that accurate.
Most clusters are large enough to be centered visually to the nearest one or two minutes of arc.
Regards, Rob.
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