All carriers in any alliance usually list themselves first, and then the others alphabetically. For example, if you go to an Iberia VIP lounge, you'll see a sign with all the OneWorld carriers listed, but Iberia will be at the top. This is most probably what's going on here. Take a look at your third example here. This Mexicana a/c has the Mexicana logo at the front, then all the others go backwards in alphabetical order, until they reach Varig, which starting with V, is the lowest letter and therefore shown last, unless on a Varig aircraft, where it would be shown at the front.
The same was true with the earlier style, but Varig had not yet joined, leaving United the carrier with the lowest-placed letter and therefore at the back of those a/c which weren't its own.
Hope this makes sense!