In reply to Postmanpat:
Although it often is dependent on tone and timing, I often take it the other way i.e. not at all passive or in anyway a concession, but rather as a declaration that they have applied some effort in reasoning their opinion and are challenging you to justify why what they _thought_ might not be correct. It is quite distinct from an opener such as as "I was under the impression" which tends to show that they are trying to have a discussion rather than champion their own view. It can also be a bit passive aggressive as it is putting someone in a position of having to possibly tell them that the cake they claim to have baked is a crap one whereas it isn't so personal when you tell them the cream on the cake they chanced from the discount shelf has turned. Sorry for the poor analogy, quickest way I can think to describe it.
When it is used as a genuine admission of possible ignorance on a matter there are usually lengthy pauses as though to signify that they are doing the thinking for the first time so it's still a bit of a fib.
When people use it in a semi confrontational manner as though they really did think through what they obviously did not, in an even cursory manner, it has the automatic result that it colours my ability to have much faith in any of their other reasoning. That's probably me being a bit of an arse but as a stereotype it often works well......
