In the mishnah Keilim 29:2, we see a list of items and an indication of how many can be attached to each other and still be considered joined with regards to tumah and hazayah (sprinkling for cleanliness from tumah).
שָׁלֹשׁ כְּסָתוֹת שֶׁל צֶמֶר, שֵׁשׁ שֶׁל פִּשְׁתָּן, שְׁלֹשָׁה סְדִינִין, שְׁתֵּים עֶשְׂרֵה מִטְפָּחוֹת, שְׁנֵי סוּבְרִיקִין, חָלוּק אֶחָד, טַלִּית אֶחָד, קְלֻבְקָרִין אֶחָד, חִבּוּר לַטֻּמְאָה וְלַהֲזָּיָה. יָתֵר מִכָּאן, חִבּוּר לַטֻּמְאָה, וְאֵין חִבּוּר לַהַזָּיָה. רַבִּי יוֹסֵי אוֹמֵר, אַף לֹא לַטֻּמְאָה:
Three woolen pillow-covers, six linen ones, three sheets, twelve handkerchiefs, two pant-sleeves, one shirt, one cloak, or one winter-cloak, can be regarded as connected in respect of both uncleanness and sprinkling. If they exceed this number they are regarded as connected in respect of uncleanness but not in respect of sprinkling. Rabbi Yose says: not even in respect of uncleanness. [from Sefaria]
However, when Rambam codifies this mishnah in Hilchot Parah 12:10 (12:7 in some versions), he separates the list of items into two, and omits pant-sleeves (סובריקין).
המחבר שלוש כסתות של צמר, ושש של פשתן, או שלושה סדינים, או שתים עשרה מטפחות--הרי אלו חיבור לטומאה ולהזיה; יתר על כן--חיבור לטומאה, ואינן חיבור להזיה. חלוק אחד, וטלית אחת, וקלובקרין אחד--הרי הן חיבור לטומאה ולהזיה
When a person joins three blankets of wool or six of linen or three sheets or twelve kerchiefs, they are considered as joined with regard both to impurity and sprinkling. Any more than the above quantities are considered as joined with regard to impurity, but are not considered as joined with regard to sprinkling. One cloak, one garment, and one klubkerin, are considered as joined with regard to impurity and sprinkling, even if they are very long or very wide, no matter how large they are. What is a klubkerin? Two garments are taken. Cotton is placed between them as a lining and then they are sewn together as one and worn as an outer garment for the winter. [translation from Chabad]
Why does Rambam omit pant-sleeves while listing all the others? Why does he split the list into two: the ones listed with numbers greater than one, and those listed with the number "one"?