We are using a CommPlete 4000, and a user reported connecting at 24K, but the logs said she connected at 31200 V42bis. Do you know why this would be the case?

The remote modem is reporting its download speed and the host modem is reporting its download speed. These modems are asymmetrical. The upsteam and downsteam speeds can be different. The modem might connect at one speed initially, but it might shift up or down to a different speed due to phone line conditions.