Not sure if this will clear things up or muddy the waters...
...but here goes:
+/-3dB is the variance from a test level of output within a defined range of frequencies. If a sub is rated to play "flat" from 25-150Hz +/-3dB at a given test level of output, all frequencies within that range will play at roughly the same level, with a variance no greater than 3dB louder or 3dB quieter.
Any peaks within that range will not exceed +3dB relative to the test level of output; and the extremes of the range (25Hz and 150Hz) will be at -3dB relative to the test level of output.
According to the test results I linked to, the JBL appears to play "flat" down to 25Hz (or 25Hz @ -3dB).
The Sony, possibly, was hitting 25Hz at a lower dB level / greater variance (say, 25Hz @ -6dB or -10dB), meaning that the range across which it played "flat" did not extend down to 25Hz @ -3dB (it may have been 30Hz @ -3dB or 35Hz @ -3dB).
IOW, the Sony might have hit 25Hz, but not at the same level of output as the JBL reportedly does.