If one digs into the numbers it's probably less than that (not a huge amount but certainly less). Unions get to report how many people are in their "bargaining unit" which is why many who are not dues-paying members get counted. The downside is that often non-managerial people who want nothing to do with the union get impacted - and even operational decisions get impacted - by the latest union case of "panties in a twist over something."
Anecdote: In an issue raised over re-writing a job description once (as a non-manager type) I once had the local union president (who was a national VP) tell me straight out in front of the whole room that they really didn't care about white-collar technical folks because they thought "you make enough money as it is." (Sound familiar?) Their interest was theoretically in the blue-collar guy, who actually probably made more than I did.
They have made themselves largely irrelevant which is why many states have gone to laws that protect their workers from having dues deducted if they choose not to be a member. Don't know about elsewhere, but that REALLY got the unions' panties in a twist in my state.