If you mean before windows loads you're getting higher scanrate video modes (31kHz+ resolutions) - that's normal. As windows boots it hands control of the display to crt_emudriver. Before that handover occurs, video modes are pulled from the bios of your graphics card, and those are not 15kHz by default. If you're using a desktop card you can look into Atom15, also by calamity. But search on here first to see if anyone is having an issue with Atom15 on your card, some work and some don't. Atom15 patches the bios of your GPU to only allow 15kHz modes. Or higher if you choose those.
If you mean once windows loads you're still not getting 15kHz, plug in your LCD and stop and disable the windows service called "AMD External Events Utility". And turn off fast startup in windows. Then go through VMM again and setup your modes, and now try restarting.
To use the LCD but leave the 15kHz modes in place, you can try simply plugging the LCD screen into a different port of your graphics card. If you have to use the same port as for your CRT, you will need to turn off edid_emulation as Calamity says.