At best you are measuring the circuit you are probing with a ground that is referenced to your scope ~ So you measurement is off by the voltage drop induced by the probe cable. At worst your probe is just an antenna and your picking up a lot of additional noise.
How ever because you probe does not even cover the frequency being measured it may look better then it really is. You need a probe that works over the frequency range being measured, and a probe which is grounded as near the probe point as possible.