Are we starting sweet potato slips the hard way?

I'm definitely not an expert, I have only been growing sweet potatoes a couple of years. I have found it easiest for me to plant the sweet potato in a shallow pot. This is very easy, and produces lot of slips . I'm actually getting slips from the same sweet potato I used last year.  
I wanted to share slips, and didn't know the name of the sweet potato I started last year, so I started one in the house with a heat mat to get an early start. It's working great.
My daughter bought a couple of sweet potatoes to eat, and they were sitting on the table not far from the grow light. They are growing slips. It makes me wonder if the soil and watering isn't necessary.
You will see the pictures and say wow huge! difference, why is she even questioning the process? Keep in mind the one in the soil were on a heat mat, and have been planted for 6 or 8 weeks??? The volunteers haven't been on the table that long, so not fair comparing them.
One thing is for sure I will have plenty of sweet potato slips.

I start mine by taking a tuber and putting it in a milk carton, laying down, with a bit of soil, and then flooding it every so often. I do this outside== no mats or lights. if it were cold, I'd probably do it under a grow light in my office.
Just letting it sprout "au naturel" doesn't give you a lot of leaves, just leggy stem.