Veryexclusive - how do they do it?

[Bumblebzz]

Just curious really.
I found a dress in Reiss last week that I thought would be perfect for a wedding next month, however even in the sale it was still £125 (from £179 I think). It was the last one in the shop and the lovely assistant had admired it on me so when I checked the price and realised it was too pricey for me she said she would check and see if she could knock anything else off it. She couldn't so I reluctantly left it. A day or two later I thought I'd check online in case they'd had further reductions, they hadn't, but my online search returned a match for the exact same dress on very exclusive.co.uk (I'd never heard of them before but part of very I think) but it was £72! I took a bit of a chance and ordered it and it IS the exact same dress so I'm delighted.
My question is though...how do they undercut so much? (and should I be buying everything from them in future if they are that much cheaper?!) Or maybe this was a one off..