Everyone can say what they want, but I asked the actual designer of the Leathernck tartan, and this is what he said (I take it as authoritative):

"I tried to log in but it gave me problems. my user name is tartanmarine. With a FT management job, I don't have time to mess with it.

In any case, there is no USMC Tartan. Can't be unless the USMC approves. I'm the designer of the Leatherneck Tartan, with input from Rory MacLeod of the Scottish Tartans Society. I submitted a design to the STS back in the 1980s. Rory suggested two changes, one of which I accepted, one of which I changed further. The pictures just show weavers using different hues.

Some dealers refer to it as the USMC Tartan, helps sales, but incorrect.

It is an open tartan, anyone may weave & sell, no royalty to me. Just honor to my Corps."

As I said earlier, there really is no "USMC" tartan unless someone else registered it or the Authority decided to name it that and change the name of one of the colors used, which it appears they did. Piperdbh is right; they're the same with varying shdes of colors, and, it looks to me, some weavers decided to monkey with the thread count on some of them. I guess that means they're not woven a true Leatherneck, but a slight variation of it.

Surely we've all seen other tartans which were not exactly the same shade before this. If not, do a little comparing at the next clan gathering you attend.