You've got a variegated S. trifasciata that's reverted to being plain green. It's fairly common.

Why does it revert to plain green? Grow conditions or ??

Reversion is the result of random mutations, but can be influenced by the environment, for example, by the presence of mutagens. This sort of variegation is caused by chimerism. The cells in the meristem which differentiate into the leaf margins carry one or more mutations that render them incapable of producing chlorophyll, so the edges of mature leaves are yellow. Sometimes the mutation resulting in variegation is unstable and, from the cell's perspective, easy to fix. Other times other mutations which occur randomly and can restore chlorophyll to these cells, resulting in reversion to an all green phenotype.

Those are beautiful.

What soil mix are you using? Is it store bought, or something you make?

I agree with stush. I have seen the same thing many times

@Stush Good point, I forgot about that.