Frequency response refers to the range of frequencies a sub can play within a given sound-pressure variance (usually +/-3dB). So, for example, a sub rated 20-120Hz +/-3dB will play "flat" between 20Hz and 120Hz with a variance in sound pressure of no more than 3dB higher or lower.
It appears that your sub's FR is 27-140Hz +/-3dB. The manufacturer appears to have added a -6dB point of 20Hz to indicate that the sub will extend down that far, but the 6dB decrease in sound pressure means that output at that frequency will most likely be anemic compared to output between 27Hz and 140Hz.
a upward change of 3db is a doubling of volume.
so yours off 6db at 20Hz...guess what....
A change of 3 dB implies doubling of power, assuming the driver stays linear, or doubling the radiating area without increasing power. In the midrange 3 dB is about one notch louder to most folks. Doubling perceived volume in the midrange requires a 10 dB increase and 10 times the power assuming the speaker stays linear. Because of the equal loudness curves, which show how human hearing differs at different frequencies the perceived difference of a 3 dB change will be larger in the bsss than it is in the midrange.
To the OP it looks like the mfr has made up different terms to name two different ways to describe the claimed performance of the sub. I would not fully trust either. They don't seem to fit together in the expected way.