See
http://gb.hilleberg.com/EN/faq/why-did-your-hydrostatic-head-figures-change/ for one possible source of variation.
Somewhere I've seen a piece from Patagonia explaining testing variations that I think included HH, but my powers of Google-Fu are weak this morning and I can't find it, but it suggested different labs and different machines would give you different results. In other words, it's not quite as standardised as we might like.
Marketing departments like to quote numbers because they're something they can point at and say, "Look, ours is better", but they don't necessarily relate to actual real-world use and they try and reduce a complex engineered construct to a single material characteristic. Goretex is "guaranteed to keep you dry" on the basis of a 20K hydrostatic head, but if you put on a Goretex jacket and go out in wind blown heavy rain you'll get wet at the cuffs, hem and neck, their "guarantee" notwithstanding.
A tent is an elaborate construction. It's going to leak through the zips because they're not waterproof (waterproof zips on jackets have been a Holy Grail for years, now we've got them it turns out they weren't good as we hoped and in many ways storm flaps are more flexible and consequently better in actual use) and if there is a failure of waterproofing in the fly it'll almost certainly be on the seams. Seaming is a combination of seam construction/design, quality of sewing machine and experience of the sewer, but that doesn't distil to a nice figure like "5,000". The seams on my Hilles haven't been sealed, but are well enough made that they still don't leak. Why no seam tape? Seam take adds weight and weakens the base fabric with heat ageing, I prefer a well made seam.
Your extra several hundred pounds goes in to stuff like QA, wilderness testing, bombproof seams made by expensive sewing machines that minimise needle damage run by very experienced workers that only make tents, as well as careful material selection.
Pete.
Often seen riding a bike around Dundee...