Hover over the information (i) icon next to a dimension. This shows the top 5 values for non-time dimensions (and 15 for time dimensions). We used to keep those values static (i.e., the 5 values picked never changed).
Now, by default, we show dynamic values instead of static ones, with the option to turn them into static values. Other things to note:
When you hover over a dimension and click the grey right-arrow next to it, a list of its dimension items appears. Any list of dimension items usually shows the top items for the last 30 days.
If you scroll down to the bottom of the list, you will see a Show Top Items From Last 6 Months. Click this option to see top dimension items from the last 180 days.