Why does holding my car's RF remote to my head extend its range?
This is just speculation.  To confirm any of this would take some careful measurements with fairly sophisticated equipment.

Two things come to mind.  First, just raising the remote should help with its range.  Try holding it up at chin level but not near your head.

Second, it's possible that the length of your body is roughly resonant at the RF frequecy.  By feeding the signal into your body antenna at one end and with the other end loosely coupled to ground your whole body might start acting like a antenna.  With the transmitter at waiste level, your body antenna is center fed, which doesn't cause it to resonate as well.  Again, this is all just speculation.