[Spoiler... sort of?] Sri Lakshmi

[Berethos]

So why would they summon their goddess in such a way?

With the summoning, it does depend on the state of mind of the summoners at the time - when Bahamut was summoned, he was influenced by Tiamat's grief and anger.

Within the scope of the lore, it is possible that Lakshmi's appearance was influenced by that, and under more deliberate circumstances she may have looked different.

That being said, it's almost certainly a case of just kind of rolling with a "this is how the Broodmother pictured her for some reason when she summoned her in her grief" while the actual design was meant to be an homage to previous designs.