I have a question which will sound really, really REALLY TMI but...

I have just read the instructions for the GBS swab, that i'm to do myself (i'm 37 weeks but will be doing it at 39 due to my ability to make a wonderful womb) and i was a little shocked to see that its a veeja swab (done with both boys) but this one also asks for a anus to be swabbed... like... why? i understand that GBS travels from the anus to the veeja, but would there not always be an amount of GBS in the anus? i tested +ive both times with the boys, thus im doing the test so late as both boys where 11 & 10 days late so want to be able to see as close to birth as possable what my GBS status is (i am aware i can forgo the tests but DH wants it done as the boys had issues at birth) but is this normal and has changed in three years since the last birth with vic?! im a little shocked to be honest - and was getting ready to start useing garlic cloves ect to kill any GBS/thrush off... but now im just confused!