I can't speak to the thinking of the WTS for making this one time application of the 12 tribes being non-anointed people. But note the buzz word "evidently" which the WTS uses to leave an opening for changing this statement later on in their publications. (Matthew 19:28, see also Luke 22:30) If you can figure out this out-of-both-sides of their mouths teaching, let me know.
But I did find this in 2008 (before 2011 quote in my post above)
19:28—What is represented by “the twelve tribes of Israel” that will be judged? They do not represent the 12 tribes of spiritual Israel. (Gal. 6:16; Rev. 7:4-8) The apostles to whom Jesus was speaking were going to be part of spiritual Israel, not judges of its members. Jesus made ‘a covenant with them for a kingdom,’ and they were to be ‘a kingdom and priests to God.’ (Luke 22:28-30; Rev. 5:10) Those of spiritual Israel are to “judge the world.” (1 Cor. 6:2) Hence, “the twelve tribes of Israel,” whom those on the heavenly thrones judge, evidently represent the world of mankind who are outside that royal, priestly class, as pictured by the 12 tribes on Atonement Day.—Lev., chap. 16.
Also in 1987, the WTS gave this very convoluted explanation that probably went over the heads of the jw readers.
see also 2010
and even in 1964 the concept that the 12 tribes here were not anointed Christians but those who would stay on earth.
---As far as I can tell, this is the only time the WTS makes an application identifying the "12 tribes of Israel" as not being those Christians going to heaven.