Sheryl, I've grown Stevia for at least 12 years now, possibly more if I go back and check my records.
Some plants will have a higher sugar content than others; sometimes this is dependent on environmental factors and sometimes on the plant. I do lots of cuttings and it seems cuttings from a plant that is low in sugar content pass along that trait.
Some of my customers say the leaves are "too sweet", others, like my daughter, could graze on it all day long! :>) You can regulate the sweetness to your liking though by making a decoction with them; just putting the leaves in your cup of coffee isn't enough to give you any benefits.
Hope this helps.
Shoe
(Beautiful plants, aren't they?)