You have two table with eight columns. Both have merged cells in the first two and last three columns. The only difference is the number of rows the merged cells span.

When you add a new row to the bottom of the table, that row will have the standard dimensions - with no merging. If you want the cells to be merged, you have to do it manually - or you can copy & paste an existing range with merged cells. Conversely, if you insert a new row between the top & bottom rows in a vertically-spanned range, the merged cells will increase in height to accomodate the new row

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]