The Countdown view mode for tickets is used to allocate tickets into buckets of when they should be resolved. In order for this to work correctly, you must first update the standard buckets, as the default out of the box setting has an erroneous setting. The result shown will be as follows:

The reason the allocation is wrong, is that there is no bucket allocated for items that need to be resolved between 1 an 6 hours. Therefore, in the example above, ticket #105 is allocated to 6 hours and more. To resolve this, create appropriate buckets. You can do this via the three dots in the upper right corner, next to the countdown menu.

Once that is done, the tickets are put in the correct buckets.