I know this because I can rank for generics but then disappear when a niche brand is added, likewise sometimes I cannot rank for the synonym which is illogical (how can I be top 5 results for one and nowhere to be seen for less popular synonym?)
I have been pushed way down for the most popular keywords too. Maybe G determined the top sites for X keywords?
Suggy, did your rankings improve as you are updating content? I am thinking of going totally without any meaningful text that can be analyzed, it's not stopping others in my niche from ranking, and at least 2-3 with several amazing user reviews have been banished from the top after the April update.