The reason this happens is most likely because the header css changes to relative on mobile, but the fullscreen slider is still measuring your device’s entire viewer window instead of subtracting the space your header takes up. If you change the header css to be absolute, the dimensions of the fullscreen slider should match those of your device.

Caveat: This will “hide” the top part of your slider image behind the header.

If your breakpoint is not the standard 768 like below, change it to whatever your breakpoint is.

@media only screen and (max-width: 768px) {
.responsive #top #wrap_all #header {
    position: absolute;
}
}