Ducklings are easily vent sexed (look up the video on the Metzger farm site). I've never bought from Holderhead, but I see that they raise some to adults to sell and I bet they get a lot of requests for extra females as adults. If I were them, I'd be tempted to pull out some extra females for raising up and sell the "male heavy" remainder as straight runs.
I have not idea if they do this or not. I once bought 100 sex link chicks from a hatchery and found I had about 60% males. I can't prove they switched out a few of my pullet chicks for males (they would otherwise not be able to sell for as much), but it would have been quite simple to do that. Both hatcheries are for profit enterprises, so the temptation to boost the profits in small, seemingly innocuous ways must be there.
Honestly, this is the main reason I am reluctant to but bantam duckings from Holderhead. They are expensive (about $11). I can imagine buying 6 or 8 and finding I only have 1 female, making for a pair that costs more than buying adults (except shipping is even higher on adults). I will probably wait until I can find some locally rather than risk a male-heavy order.