I was able to reproduce the issue from your comment and I logged in our Feedback Portal. You can track it on the following link: http://feedback.telerik.com/Project/154/Feedback/Details/149583-fix-radspellchecker-single-letter-in-parentheses-is-underlined-with-a-red-line
In addition the check as you type did not recognize a colon as punctuation and gave an error for (b) in an itemized list. The check method did not give errors for these. I would like to see full consistency between the two methods.