I think Kuden's right. Your computer's volume slider doesn't measure decibels, but displays voltage gain as a portion of your sound card's total possible voltage gain. The volume knob on your outboard amplifier works the same way: the farther you turn it, the more voltage gain you get, and the closer you get to your amp's maximum voltage gain.
As Kuden says, the efficiency of headphones (and speakers) varies, so that given a signal input of a certain strength, the output will be higher with higher-efficiency headphones and lower with lower-efficiency headphones.
In acoustics, decibel (dB) measures sound pressure, so the sound pressure your headphones produce depends on the strength of the input signal and the efficiency of the headphones.
Wikipedia defines a decibel as "a dimensionless unit of ratio which is used to express the relationship between a variable quantity and a known reference quantity. . . . The calculation of decibels uses a logarithm to allow very large or very small relations to be represented with a conveniently small number."
What "known reference quantity" is used in acoutics decibel measurements? Wikipedia reports, "the decibel unit is commonly used in acoustics to quantify sound levels relative to some 0 dB reference. Commonly, sound intensities are specified as a sound pressure level (SPL) relative to 20 micropascals (20 µPa) in gases and 1 µPa in other media (standardized in ANSI S1.1-1994).[1] 20 µPa corresponds to the threshold of human hearing (roughly the sound of a mosquito flying 3 m away)."
Pretty neat stuff, even if I never could quite grasp logarithms!
EDIT: I'll be gabedamien is right about the -dB measurement!