Definition of in the Top 100
In our marketing stats email this is a count of the number of search keywords your site ranks for in the top 100 results on Google (only keywords we are actively optimizing you for are counted). This count only applies to those keywords we are optimizing your website for. Meaning if we are optimizing your website for 80 keywords the highest number you could get for "in the top 100" would be 80. Keywords we're not optimizing your website for are not looked at for this statistic.
Google typically shows about 10 results and then separates the rest on additional pages as seen below.