because Austen is satirising small town gossip and how unreliable it is. Meryton gossip at the beginning of teh novel made out that Bingley had 5 sisters... he only had 2. Wickham was held to be a charming angelic young man - he wasn't... but when it was revealed that he had behaved so badly and seduced a young lady, suddenly everyone changed their minds and they were all talking about how bad he was, and how he had apparently seduced every tradesman's daughter in the town...