Class A airspace

OK, I'm puzzled. Maybe you folks can help me understand this. I
looked up the definition of Class A airspace, which is "from 18,000
feet MSL to and including FL600" with few exceptions that don't apply
to the area I'm looking at. I've seen more than one flight posted on
OLC which have a high point above 18,000 feet, up to 18,500 feet. Is
there some fine point I'm not understanding which makes this legal?

Heck, I think it should be illegal to fly in conditions where you could
thermal into the stratosphere. Unless they can make conditions like
that back here in the east, where about 6,000 is the highest I've
gotten all summer.

Thanks in advance,
Ed