trop cantillation - Why do a few tribes in a list have different trope from the rest?

In the first aliya of B'midbar the chiefs of the tribes are listed. Nine have the zakeif gadol trope and three -- Yisachar, Binyamin, and Naftali -- have different trope. This isn't driven by word count (e.g. the Yosef tribes, even though they have more words, still use the same trope, just with some additions).

I wondered if there were something special about these three tribes that the trope is trying to call out. Another place where representatives of tribes are listed is in Sh'lach l'cha, and there also there is a majority trope with some variation -- but it's not the same tribes there (Yehudah and Manashe there).

Why do these three tribes have different trope in B'midbar? Is there a meaning or is it just melodic variation? And if there is meaning behind it, where can I find commentaries that address it? Most of the common commentaries (or at least the ones I have) focus on text and don't address trope.

Answer

The alternation you are speaking of is between a zakef gadol (on the many, short names) and a 'pashta'-zakef katon (on somewhat longer words). Both of these are really a zakef. When the zakef comes on the first word of a clause, or of a pasuk, as it does here, then there is no place for the servus. This is where we typically see a zakef gadol version of the zakef.

The 'pashta'-zakef katon is caused by purely musical concerns, based on characteristics of the syllables. But it is really just an equivalent of the zakef. Here is what William Wickes has to say about it:

If there is a closed syllable in the word, separated by one or more others — or, at least, by vocal Sh'va — from the tone-syllable, an emphatic intonation (a high tone, as the position of its sign above the word implies) was introduced, serving as a forebeat ( Vorschlag) to Zaqeph, in the absence of the foretone. It was known as Methiga, being like an upper Metheg (comp. the use of the term in the accentuation of the three books, Taamei Emeth, pg. 70).

Keep reading at the link for more information about the Methiga. But consider: Le-Yis-sas(?)-char. That is a closed syllable, Yis, separated by one other syllable - sa - from the tone syllable, char. So too Bin, a closed syllable, ya, a separating syllable, and min, the tone syllable. And so too Naf, a closed syllable, ta, a separating syllable, and li, the tone syllable.