It's based on nothing concrete, but I'd assume those that don't believe Northern Ireland should exist would be more likely to not vote in the Northern Irish elections.
Wouldn't they be more likely to vote for the pro-United Ireland party?
If they don't vote, then they are content with how things are, no?
They'd be more likely to vote for a Nationalist party yes, but they'd also be more likely not to vote as that would acknowledge the legitimacy of British rule over Northern Ireland.
That, or they are avoiding debt collectors by not being on the electoral register.