This is by design. Rietveld's LGTM parsing was an absolute nightmare, and often resulted in people having to post a second message saying "not lgtm" (which then turns scary red) because they didn't realize they'd typed or pasted "lgtm" somewhere in the middle of their message.

We offer the nice shortcut that a message started with "lgtm" will also set CR+1, because that is a very clear indication of intent. Otherwise, please just use the CR+1 button like normal.