What's so strange about this? After the page in 4th place one of your pages is placed right below it in the ranking scale, the next one happened to be one of your pages too... It's possible that the 6th one itself is worth to be shown in 7th..10th places, but to make the SERPs look more tidy they are displayed together.
Since you have two different pages that are related to the term and don't contain duplicate content, they are treated as separate pages the user might be interested in.
There are hundreds of keyphrases for which my websites show in more than one position. For one keyphrase though it dominates first four SER pages... G might be messed up with that one, other than that it's natural.