Because they made a point of calling it out in the story. No one ever looked at the Romulan Commander and said, "Hey, isn't that Spock's dad?" But Picard thought Tallinn was Laris at first. If it was just the same actress playing two different roles, why call it out like that? Also, the actress was *still* playing the character of Laris this season. So having her play another character at the same time is kind of an unusual decision unless there was a reason for it, like Brent Spiner playing Data and Lore.