Pasting an HTML table into the text view of the editor in WordPress will display double lined boxes even for single line rows. This has been a bug extant even with WP 4.8.
To circumvent this bug you need to add some inline style to each row. Add
to each <tr> and <th> tag.
for those rows and headers with 2 lines of text, this is not needed, it’s only needed with one line rows.
Eventually we are hoping the WP team can fix this problem so that HTML tables are less of the headache.,