It is a 75 ohm IMPEDANCE, not resistance. Impedance has both a resistive and a reactive part, and is properly represented as a complex number of the form r+iw, where i is the square root of -1 (an imaginary number). The "r" part is pure resistance, and the "w" part is the reactive part.
Direct Current (DC) only sees the "r", but Alternating Current (AC) sees the "r" and the "w". So measuring it with an ohmmeter only gives you the "r". The 75 ohms refers to the magnitude of the complex impedance (sqrt(r^2 + w^2) for the mathematically inclined).
The "w" term is often a function of frequency.
Thus you need a much more complex setup to measure true impedance. The 75 ohm value refers to how AC sees the cable.
The above oversimplifie a lot of things. See a good textbook on cirsuits for the real lowdown ;).
