That is cause by default the _top layout shows nothing, no bug or anything.
To show the stats place {$this_period} anywhere in the table_top_row_premium.html and table_top_row.html
That tag auto detects your ranking method/period
Alternatively, you can manually place different statistics, such as
{$unq_pv_0_daily}
- {$A_B_C_D} - Stats - A is 'unq' or 'tot'; B is 'in', 'out', or 'pv'; C is either 'max', 'avg', or an integer from 0 through 9 where 0 is today/this week/this month and 9 is 9 days ago/9 weeks ago/9 months ago; D is 'daily', 'weekly', or 'monthly'
- {$A_B_overall} - Overall stats - A is 'unq' or 'tot'; B is 'in', 'out', or 'pv'