...This is because you can not predict when Python will be busy garbage collecting. It also runs under the Linux kernel which is not suitable for real time applications - it is multitasking O/S and another process may be given priority over the CPU, causing jitter in your program. If you are after true real-time performance and predictability, buy yourself an Arduino! (see http://www.arduino.cc )
Note that the current release does not support SPI, I2C, 1-wire or serial functionality on the RPi yet. This is planned for the near future - watch this space!