To be fair, they also often look like a deer in the headlights if you ask them about the strain lineage, and that's likely to be a lot more meaningful than some percentage on an indica/sativa continuum. It's not like anybody has a well-calibrated sativometer or indicator taking measurements to six digits of accuracy, and strains that supposedly have the same I/S percentages can be wildly different from each other.
Brilliant
.. sativas with good amber still completely different than indica if that helps.. 
Some do give a % of ruderalis. Not sure it's worth believing tho. I would like to know how they arrive with the percentages as well. I'm assuming they guess mostly whether educated, or not. Im a "sativa" guy, and some definitely do give you energy. Typically they're the cultivars we call sativa, or sativa dominate. Not all tho... like @Bob's Auto's said the terpenes have a big role in it. I also believe each person's personal chemistry has a part to play too. Take a look at Fast Buds Green Crack. I have never got a shot of enery like I have from that, and it's listed as only 55% sativa. I did a Dutch Passion Durban Poison 100% sativa. Her bud puts me to sleep. Although that 1 does state if you use it a lot it will put you out. I have tried to stop asking for sativa effects, and ask for energetic effects now.
