I thought methane and carbon dioxide were the same thing...

Several is equivalent to Some.

Several X are Y = Some X are Y

Several in LSAT means at least 2, but not necessarily many.

Some means at least 1 but usually at least 2 lol

since several guarantees at least 2, and some guarantees at least 1,

so, we can safely say :

Several X are Y = Some X are Y

Correct me if I am wrong.

you will get it eventually