This was explained https://moz.com/blog/spam-score-mozs-new-metric-to-measure-penalization-risk and https://moz.com/blog/understanding-and-applying-mozs-spam-score-metric-whiteboard-friday
Isn't 2/7 but it's 2/17.
- According Moz pages with "external link in navigation" percent of sites penalized with this flag - without that flag are penalized = 19% vs. 7%. Hmm... with 19% i will be hesitated about this flag! Same numbers are for "large number of external links".
- Nofollow wont help too much because links are internal and external. And giving external link nofollow in navigation isn't helping. Should you ever have external link in this navigation? What is purpose of this site - low quality site with landing pages full with keywords and EMD? Should this site exist in SERP? Please note - it's end of 2015 and there are new rules for SEO shared in Rand Fishkin presentation: http://www.slideshare.net/randfish/onsite-seo-in-2015-an-elegant-weapon-for-a-more-civilized-marketer So today people are worked for getting higher CTR, avg. visit duration, pages/session even bounce rate. And single external link in navigation can broke statistics/analytics.
If everything is OK on your both sites or network of sites. But that 19% wont give me a good and quality sleep. Who's know how tomorrow algorithms will evaluate sites?