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This only occurs when you hide extra rows. In your table below the only case where you used the 'Hide' option is the right-most column, 'AR0220AT/hidden'. In all other columns you have not used the 'Hide' option, so the 'last row' is 'height' - 1.
Double-click "Mouse Selection" bar to open the dialog that allows setting of up to 8 unique mouse configurations. Also see "Lock Mouse Selection", here.
Summary
Not using 'Hide' option: 'last row' = 'height' - 1
Using 'Hide' option: 'last row' = 'height' - number_of_hidden_extra_rows - 1
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