Originally Guam appeared as part of the South Pacifc grouping, which included Australia and New Zealand. However, the problem was that to get there from these two countries required transitting through SEAsia. And since the award level -- which varies depending upon which STAR partner program you are using [or travel within that region is 25/35/50 in Aeroplan] -- is so low, it was an amazing award if you wanted to luxuriate in J or F for significant lengths of time, even cheaper than flying between the two countries and your transit point in SEAsia.
Since no STAR carrier flies directly from the South Pacific to Guam, one must connect in SEAsia with a carrier that does to the island, which I believe is ANA and ASIANA.
Therefore, it makes sense to place it in that region from which you must transit to get there. But it still makes a great award based on time in the air to enjoy front cabin service. [Unfortunately, I am not sure any carrier offers First any longer.]
There were a couple of us who noticed this when the new STAR Award charts started coming out. One of us was able to actually book the space with UA's program, but then it was withdrawn by UA. AC would never confirm space, and within a couple of days, both STAR programs amended Guam to fall into SEAsia rather than SPacific.