Connecting to ISP with 56K modem

I have a Gateway Solo 1200 laptop with a 56K modem (factory installed). I also have a IBM compatible desktop with a 56K modem. My question is, when I connect to my ISP with the desktop I connect at between 43 and 48,000bps. When I dial up with the laptop using the same phone line and dial-up number, I connect at only 19 to 24,000bps. Why wouldn’t the laptop connect at the higher rate?