The keywords shown in Tabtimize are found, assessed, and scored in different ways.
When Tabtimize analyzes a page, it tries to extract the words that have a special meaning to the content. This is assessed based on dominance, relevance, and centrality.
Each keyword is assigned a score in accordance with that keyword’s performance of the three rating requirements mentioned above. However this score is not disclosed for all keywords, only for the once relevant for your page, which can be assessed in the LRS break down.
When you hover over the keywords data point you can see a black box containing with keywords. The keywords in this box are displayed in a descending order with the keyword with the highest score first.
This means that they are not necessarily displayed in an order that is relevant to your page, but in an order that shows their importance to the particular page they are displayed for.
However, the keyword that is relevant to your page and with the highest score will be displayed as the default one.
Disclaimer: These keywords are not necessarily the same keywords that Google or other search engines use to rank the page with.