I have looked around, but cannot seem to find out why. On SATA drives (2.5 and 3"), there are variable lengths to the copper contacts on both the data and power ports on the drives. These variations do not exist on an mSATA.
Is there a reason why these variable lengths are on all SATA plugs?
The longer pins are ground lines ("GND")...
Data:
Power:
Ground lines are slightly longer than data/power lines on connectors that are intended to be regularly plugged/unplugged (especially in the case of hot-swap capable interfaces, like SATA) to ensure that the ground lines are being connected before any data/power lines are. This helps prevent electrical discharge into the device when attaching/removing it.
mSATA is an internal, clamp-down, 'permanent' connector type, so it's not intended to be "plugged" and "unplugged" regularly (by end users), and definitely not while the power is on. :)
I should have looked just a little more. It is for the ability to hot swap.
While full size SATA drive are hot swappable, drives like mSATA are not intended to be. Thus why the SATA has staggered and mSATA does not. Makes sense.
Most modern hot-swap methods use a specialized connector with staggered pins, so that certain pins are certain to be connected before others. Most staggered-pin designs have ground pins longer than the others, ensuring that no sensitive circuitry is connected before there is a reliable system ground. The other pins may all be the same length, but in some cases three pin lengths are used so that the incoming device is grounded first, data lines connected second, and power applied third, in rapid succession as the device is inserted. Pins of the same nominal length do not necessarily make contact at exactly the same time due to mechanical tolerances, and angling of the connector when inserted.
This image from same source lines up the contacts pretty good. Shows length and attribute.
And this gives a good view of pin out to the extended/shortened contact locations:
