Google-search – Why is Google asking “Did you mean…” when it’s the same word

I don't know about the technical details of the implementation of those other sites. However, if there are sites that don't use account or cookie prefs to know what you normally search, I would expect nothing less than different results.

Google has been personalizing search results for years to give you information tailored to your needs. Recently, Google search results have been more explicit, displaying search results based on what people in your friend network work have shared. Also, see ratings and reviews on Google Places, for example, where your ratings and those of your friends modify your results giving you better, more personalized information.

Perhaps this is because when linking to Google+ members the + is a prefix to the member name.

See, e.g, Google Kills Its Other Plus, and How to Bring It Back on Wired:

Google wouldn’t disclose exactly why they phased it out, though it seems obvious that they’re paving the way for Google+ profile searches. When Google+ launched, instead of adopting Twitter’s @reply syntax, they coined their own format for mentioning people — adding a plus to the beginning of a name — triggering the future conflict with the + operator.