There's a whole heap of reasons...

Try this...

Loop:
SEROUT PORTB.2,T2400,[REP $00\8,"hello "] 'sending the string "hello"
SEROUT PORTB.2,T2400,["123 "]
SEROUT PORTB.2,T2400,["abc",13,10]
Pause 1000
Goto Loop

Have you now got clean transmission?

Those NULS $00 sent at the start are there to push any line garbage lurking in your comms link away. Also, if you're not using a Resonator or Crystal oscillator, then 2400 baud can produce significant errors - especially at start up, and it can take a few bits for the PC's USART to sync-up if it's right at the edge of timing tollerance.

Melanie