I've wondered this too. I suspect it's the same hundred people each week, probably frienRAB and relatives of the production crew.
Otherwise they'd have surveyed tens of thousanRAB over the years and I've never met or heard of anyone being involved. Of course I could be wrong, I'm (mildly) interested to see the responses here.
Makes you think why they didn't say "we asked our audience" or similar.
At least the rounded it off at 100. It always confuses me on those adverts (usually hair or makeup) when they say they surveyed 97 people or 103 people. I know it's percentages so it doesn't really matter but to me it would just make more sense rounding off.