Honestly, this is one I don't think requires too much thought - there are more Primals than elements because, in a series-meta sense, there've been more genjuu than elements for a very long time.

The only title to have more or less exactly one summoned monster per element was FF3 - that had Chocobo (for wind), Shiva, Ifrit, Titan, Ramuh, Odin, Leviathan, and Bahamut. (An interesting note is that, as of ARR, every single one of those summons has been introduced as a Primal sans Chocobo, who is instead a species :V and has been replaced in the line-up with Garuda, who has become an "iconic" summon for the online FFs specifically.) Even here, Leviathan and Shiva sort-of share an element and then FF4 ballooned out the list substantially - not even counting the item-drop summons, Sylph, Mist Dragon and Asura were all added to the list. 5 added even more, including icons like Carbuncle, Phoenix and Golem.

In 6 it got completely crazy-go-nuts; there was a summon for each element up to and including "elements" like poison or healing (the latter of which had multiple summons), there were multiple tri-elemental summons, multiple "ultimate-element" summons, of which Bahamut wasn't even the strongest... et cetera. At this point, if they continue to crib summons from games past as Primals, we're going to end up with a lot of repeated elements, simply because it's already an established tradition for there to be a number of such repeats.

From an in-universe perspective, though, it's very possible Moose's theory is correct - or that each continent has its own distinct summons for each element. They do seem to be local rather than global "deities", after all - but then we know so little of Othard or Ilsabard that it's a bit hard to say either way.

But yeah, I wouldn't think too hard on it simply because there've been a lot of summons with crossed elements for a very long time, franchise-wise, and there's going to be crosses here if they want to use more of the classic, remembered ones.