First and foremost game play mechanics.
In Morrowind we have this big book about necromancers building skeletons and the likes, we have this Mechano instruction manual to construct a Dwemer Centurion, yet you can summon them. Which doesn't make sense. This suggest that summoning is used as a substitute for creating one in advance.
In Oblivion we have groups of Necromancers and groups of Conjurers, the Necromancers decidedly stick with summoning Zombies, the Conjurers stick with Daedra. If it were all the same thing, if it were all just ordinary conjuration, the distinction wouldn't exist.
In Bloodmoon we have spells that are named Call Wolf and Call Bear, not a bad spell thematically as Solstheim is riddled with them. Yet rather then calling one, it's summoned. This again suggests that it is a game play substitute.
Just about the only spell that actually has a back ground that suggest something is actually being teleported in is the Summon Fabricant spell. It's attached to Barizildirs mazed band which can be used for just that, teleporting stuff.
Every time something comes in the form of Summon X, and X is not from Oblivion there is a clear suggestion that X is also not summoned. But rather constructed before hand, near by and called or very rarely by use of dangerous artefacts, teleported in.
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And done three times over already. Hmmpf.