The Hanukkah Torah reading for weekdays (per Ashkenazic custom outside of Israel) is two days of the princes bringing offerings from the section of Numbers 7 (e.g., on day 3 of Hanukkah we read of the prince who brought offerings on day 3), plus the prince who brought offerings on day 4). On Shabbat of Hanukkah (when not also Rosh Hodesh) we read the regular Torah reading for 7 aliyot, and the maftir reading is just one prince, corresponding to that day of Hanukkah (e.g., if Shabbat is day 3 we read of the prince from day 3 only). Hebcal.com right now is showing that the Shabbat of Hanukkah this year we read from Numbers 7:18-29, which is two princes:
I'm guessing the algorithm is just using the weekday Hanukkah custom to figure out what the Shabbat maftir reading should be, but that seems to be incorrect. For the source of the Hanukkah reading, you can check Shulkhan Aruch OH 684:2 (note 6 in the Mishnah Berurah specifies only one prince in the maftir).
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