There's something weird happening on UPN's Girlfriends. Although there have been no murders or voted-off-the-island cliff-hangers, young-adult ratings for the show are up by 19 percent, after sinking 21 percent last season in fact, ratings for all of UPN's Monday-night comedies have seesawed similarly. Earlier this year, UPN investigated and came up with its own explanation: The viewers were there all along. The network says Nielsen undercounted black women, a large segment of UPN's audience. But Nielsen sticks by its numbers: ''UPN is [assuming] those homes would be watching UPN,'' says rep Jack Loftus. ''You don't know what they'd be watching they could have gone out to dinner.'' But presto viewers have returned. ''Beginning [late] last season, we saw that [the sample of black women] had now come in balance with population estimates,'' says UPN and CBS' exec VP of research, David Poltrack. Now, UPN, how about tackling that Sex, Love & Secrets debacle?