If you are Strep B positive then you will always be a carrier, but what is important is that the bacteria haven't colonised at the time of birth (I think that's the phraseology) so although you will be +ve always, you need the swab to come back negative in terms of colonisation. This doesn't mean it isn't picking up the fact you are a carrier of Strep B, it just means that you are not in the risky zone when you are swabbed.
The big issue is WHEN to swab as it takes 2 weeks for the bacteria to become problematic, thus if you swab to early or too late the results can be unhelpful - or, at worst, useless.
If you are likely to go to term then swabs are usually taken at 39 weeks or, if you are fortunate at 37 weeks and again at 39 weeks.
It is the only way of knowing if you are at risk so you need to ask for a swab and not depend upon having no 'symptoms'.
There is some disagreement as to whether a high vaginal swab or low vaginal one is best but any swab at all is better than none.
Hope that helps. As I say, as far as I am aware, dangerous colonisation is not detectable due to lack of symptoms unless you have the test.