Q.Why Does My Mint Grown From Seeds Look So Different From Pictures Of Mint?
I grew spearmint from seeds (very difficult) and the leaves look morphologically quite different than any mint I’ve ever seen. It smells like mint, and has the typical square stem, but the leaves have rounded lobes/leaflets, not the typical serrated edges of typical spearmint.

Certified GKH Gardening Expert
unfortunately, your photo did not come through. I cannot see the leaves in question.
It sounds like a case of cross pollination. If these were planted from openly pollinated seeds, or even your own previous crop, then they could be a cross between the original mint, and many other from the same genus and family.
In short, from seed, it is likely that you have a completely different, but still usable plant.
Another possibility is that they are, simply, too young to express serration.
Here is an article to help you with the care of mint: https://www.gardeningknowhow.com/edible/herbs/mint/how-to-grow-mint-plants-in-your-garden.htm