The radio waves leaving the transmitter form a pattern of intensity in the space around the antenna, and the intensity usually varies by direction   In directions where the intensity is higher, the signal will be detected at greater distances by the receiver.  The shape and size of the antenna and the frequency of the wave, in part, determine the intensities and directions, but so does any other conductor or dielectric material in the path of the waves--such as your head.  You are likely refracting the beam pattern and focusing it slightly.

Remember that the fob works by sending a light signal (typically microwaves with wavelengths far greater than those of visible light).It radiates the signal in all directions, so that most of its energy goes to warm the universe. That may sound very charitable, but it is not really economical.

Now, most reflective objects reflect light because they are good electrical conductors, for example the sliver or aluminium backing of mirrors. Your head is a modestly good electrical conductor, far better than air anyway (I hope!) and it is thick enough to be a modestly good reflector of that wavelength of microwaves. If the light were visible, you could imagine getting an equivalent effect by holding your fob against a muddy white plastic model of a head. It would not be a perfect reflector, but it would redirect at least some of the light that otherwise would have got lost in the opposite direction.

You could get a similar or better effect with a reflector of metal, but for most people that is easier to mislay than your head. Or so I understand. Feel welcome to experiment.

Cheers,

Jon

Remote keyless systems operate by broadcasting radio waves on a particular frequency. Most RKEs work on 315 MHz in North America and Japan, and 433.92 MHz in Europe. - wikipedia

A key fob might work better when it's higher off the ground.

There might be fewer items in the way of the waves, or less confusing reflections, ghosts, to interfere with the decoding of the incripted signal.

A 300mhz wave is 1 meter long. So if the difference in the distance between the original signal and the one reflected off the ground is 1/2 meter, the maximum phase cancellation occurs. Increase that to a 1  meter difference, and it's back in phase again.

Try it at arms length from your head as opposed to being close to your head. It may be that when you raise your arm you change the orientation, twisting it a bit so the stronger signal coming from one edge or face of the fob is then pointed at the car.