They don't work because the lavender theme seems to have some overwrites to fix 100% not fitting in some browsers:
Because of the .home ... it's more specific and overwrites the default bootstrap classes.
To make your own ones working you have to use sth. like this for now:
ping @psychobunny maybe he'll fix that for standard bootstrap classes (would need to include all col-md and col-lg classes definitions into theme thought).
PS: you can see/change the bootstrap settings in lavender here, so it's lg: 1200px+, md: 992px+, sm: 768px+, xs: 480px+
But beware that this affects all columns on each page not only on home... If you intend to only change home you should rather overwrite them with .home .col-... as in above code-block