The usual reason for this is that the table is actually split and
consequently is more than one table. You can test this by selecting a
column. If the selection doesn't go all the way from top to bottom, then
you're dealing with multiple tables. This can happen if a table (or a
portion of one) becomes wrapped, so check the Table tab of Table Properties
to make sure that wrapping is set to None for all portions of the table.
Also, in material pasted from the Web, it is very common to have nested
tables, which can also cause problems.

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Suzanne S. Barnhill
Microsoft MVP (Word)