HP/Dell I believe may be using 12V power supplies whcih connect to the motherboard and then 5V + 3V is then split out from the motherboard. Hence whay they use smaller connectors from the PSU to motherboard.
As has already been said, recent HP and Dell machines use a 12 Volt only PSU - this connects via one of the 6 pin connectors on the board, the other 6 pin connector then normally has a cable that has the SATA connectors on, as they are fed by VRMs on the motherboard.
I came across one of those earlier in the year in a HP Pavilion 590 Desktop machine, as stated above it's a propitiatory (at the moment) system, that uses only 12v and then has the motherboard distribute 5v and 3.3 as required, it is supposed to be the way most will be going.
I did mange to come up with this, which is not fully tested, as I bought a genuine PSU upgrade for it because of size/fitting issues as well in the silly case they used!
Forgot to say, also when researching that PSU cabling, I came across reference to a different "P2" plug wiring (image below) which i was not able to identify (because mine did not have that)
Actually the reason they, HP and others have moved to this is because they have been forced to via power saving initiatives that affect companies of this scale.
By mandating only a 12V rail from the PSU, it's easier for PSU manufacturers to produce an even more efficient PSU than the various 80plus standards (or at least cheaper to get that efficiency), and means that the regulation for the other rails (e.g. 3.3v / 5v) is then provided by the motherboard manufacturer and therefore sized exactly for the devices that can be connected to that motherboard (e.g. really simplified example but if there are only 2 sata ports on the board, the 5v rail will only be sized to power 2 sata devices)
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