the "+" is for combined keywords, e.g. rio+de+janeiro. If you enter "rio de janeiro", it is transformed to rio+de+janeiro. I suppose searches ignore the individual keywords, so someone looking for "janeiro" will not find those with rio+de+haneiro, but I can not tell for sure. Why this happened to you, I do not know.
Remember that they suggest to use a minimum number of keywords and the five more important first.