Because this is triggered when you have an excessive amount of related phrases.
If "flipping widgets" has 100 phrases that Google considers related, there is a certain number of those phrases which can be present on the page before this filter can be implemented. It seems to depend on a number of factors though; it's not just a number like 41 related phrases is ok, 42 and above is not for example.
To understand the filter you have to understand what they are trying to combat. Previously there was a number of prevalent spammers abusing google's lsi/lsa based ranking criteria quite well; especially in conjunction with trusted domain parasitic SEO.
Even so, it seems this filter is really going awry in many cases. I find it surprising that it isn't effective enough turned down a few notches.