OK, first off, a magnetic mount antenna is not a dipole... If you extend the antenna to 1/4 wave AND connect a quarter wave wire to the shield of the coax, then you can make it one... But without a matching quarter wave ground element, it's not a dipole.

Magnetic mount antennas are supposed to be magnetized to something, a metallic something, a metallic something that is large enough to function as a ground plane... Like the top of a car, or in a pinch, a cookie sheet. Have you magnetized it to something? If that something is electrically about 1/4 wavelength in diameter, then you can use the quarter wave length with the antenna again... Other electrical sizes result in different antenna types.

When used with bigger ground planes, ones big enough to be viewed as electrically infinite by the radiating element (Really not that huge, maybe a wavelength or two, which is only like the size of a family sedan at UHF frequencies), you can use a 5/8 Wave antenna. It's longer than a dipole, but it works really well with an appropriate metal ground plane.

There are other wire antenna types... There is an end-fed half wave, which might be the configuration you accidentally measured (Or a harmonic thereof).

But if the coax is working... As in, the outer shield is preventing the inner core from resonating... I don't think the coax cable is the source of your theory/observation interface problems; Your coax shouldn't be causing THAT much of an error... Maybe just a few percent.