I haven't looked into how each state's laws were interpreted, but what I think it's really showing is the minimum age someone can be to have sex with an 18 year old.

Some states allow an age gap of one year, so they'd show as 17, but that doesn't mean a 30 year old can be with a 17 year old legally.

Other states allow an age gap of 18 months, so they'd show as 16.

Yes I think this only applies if one of the parties is considered an adult. At least that is how it is in Canada. 

grayruby

charts_and_numbers

Age of consent for different states

carter

I don't think those are all complete on/off cutoffs. I believe many states have formulas for how much older one party can be, until some other broader age of consent is reached.