Looks like your jpac is not getting a 15 Khz signal so it is halving the signal.
"If required, can divide a 31Khz VGA signal in half so that it will display a stable picture on a 15Khz arcade monitor. Useful for checking boot-up progress and configuration (not for application use). Automatically stops dividing the signal when it detects a correct 15Khz rate is being send by the PC."
Probably something not setup correctly in soft15 or the card to give you a 15Khz signal. Sorry no experience with soft15.