By placing one sub near a corner, you are essentially turning the entire room into the speaker enclosure.
Yup, it's boundary loading. For every surface you add within a 1/4 wavelength of the enclosure, you net a 3dB gain in output as each surface acts as an acoustic "mirror" doubling the number of virtual drivers. So:
sub hanging in mid air = 0dB (whole space loading)
sub on the floor = +3dB (half space loading)
sub on floor and against 1 wall = +6dB (1/4 space loading)
sub on floor in corner against 2 walls = +9dB (1/8th space loading)
In theory if you had a horn shaped room and placed the sub in the small corner where the ceiling met the floor, you'd have +12dB more output compared to whole space.