I wonder if a lot of people at your LHBS use sodium met, and heaps of it, as a sanitiser. As in some of the early posts, this is a way to remove chlorine and chloramines.
I've had many good chats with my LHBS owner who swears by it, but in the past when I've used it as a sanitiser I've had 50:50 results... probably worse than that actually. Since moving to Starsan I've had no issues with sanitation.
SO... I thought maybe I just wasn't using enough. If people dump a fair bit of powder in or a few tablets, swirl it around, then add their tap water it has likely sanitised and also left enough to dechlorinate. If in an area with chlorine only, one or two tabs is probably enough but this might not be enough for chloramines. I now use vitamin C (sodium ascorbate) which rocks.
Regarding your AG being ok, it I'm not sure how you do it. There is a stark difference between chlorine and chloramine - in terms of persistence. There is a heap of chloramine in the Gladstone water, but the Rocky boys up the road only have chlorine (gassed). I made a bitter a few batches which was totally ruined because I forgot to dechlorinate my sparge water (2/3 of the water) and I'm now particularly sensitive to bitter, plasticky, bandaidy chlorophenols. So if your local water uses chlorine rather than chloramine, your sitting/heating prior to dough in may have been enough. Your pouring of the water into the goo + adding yeast shortly after may not have been.