Variability in Mango seedlings isn't unusual. Nothing to worry about.

I figured the deference in leaf structure and growth pattern maybe attributed to the two cultivars that makes up Kent mango, which is Brooks and Haden?

Maybe, and maybe not. With grafting you get a clone with identical genetic make up of the parent plant. Pollination is like a lottery, there's a mix of genes that come into play. And if you consider the possibility of pollen being brought in from a different variety by an insect then the chance of variations increases drammatically.

Kent is apparently a monembryonic mango. So any seeds would be a result of cross pollination.