Those calculators use maximum values for everything - they assume 15W or similar per HDD, whereas in reality they typically pull 7-10W at full load. They'll take the CPU's full TDP, which only applies if you're at 100% load on all cores.
They then assume that all of this is working at the same time: in reality it's very rare for your CPU, GPU, sound card, HDDs to ALL be raped at the same time - typically your system at high (stress test) load over a full minute will use something like 95%/60% CPU/GPU, or 80%/90% CPU/GPU, with little HDD involvement). So they add all the numbers together, which isn't necessary.
They also add in a margin for error, a margin for expansion (most people add a few HDDs and fans over the life of a PC), some extra for USB devices, a margin for error on those, a percentage because of capacitors losing capacity, and some even assume that you'd rather pay more for a more powerful PSU just because it's more efficient at 80% load, so add 25%.
And don't forget that your PC will very rarely be at high load, and that your PSU can usually supply a peak load above it's nominal load. Typically 125% of rated performance for short bursts.
In reality a mid range-high end, single GPU PC typically draws something like 250-350W. Add in a little extra for overclocking, the PSU losing capacity over time, upgrading your GPU later and you arrive at the figure most people go for as a minimum: 450W, with most people taking a 500-600W PSU just for the extra headroom and peace of mind.
Even a dual GPU overclocked system typically only draws 500-600W at load. Again, people tend toward a little peace of mind and headroom (often these PSUs get used for more than one build) and so end up with 700-800W PSUs.
The 1000-1500W PSUs are only worthwhile for heavily overclocked, water cooled, dual or triple-GPU systems which need the extra power security.
