Helvetica Neu 10 pt is the default text style for Numbers tables.
Setting a different style to one, some, or all cells does not make that different style the 'default.' Here's an example of the effects of three different sets of actions. All cells in the table had been set to display text as Jazz LET, 12 pt. The original content of B3, C3, and D3 was the same as is shown in C4. For each of the cells in row 3, the procedure described below was carried out, then the text shown was typed into the cell.
When you Select a cell, then "Cut" the cell, the cut includes the text AND any style that has been set for that cell, and the cell reverts to the default style. See cell B3 in the example.
If you Select the cell, Copy, then press delete, the text (and style) are copied, the content (text), but not the style, is deleted. See C3.
If you click on the cell to select it, click again to place the insertion point in the cell (to edit the text), then Select the text and press delete (or Cut), then the text is removed from the cell, but the style remains. See D3.
Regards,
Barry