Well, that's the why?
All you need's a How to Fix.
Assuming you want a fix, (>> I am just curious about PIC speed.)

If you need the 2 pulses to transition at the same time, you could do something like this...
Code:
PORTB.7 = 1
PAUSEUS LX * 10                ; adjust to PULSOUT's 10us resolution @4mhz
PORTB = PORTB & %01011111 | %0010000    ; R-M-W both pins at the same time
PAUSEUS RX * 10
PORTB.5 = 0
Of course, this just shifts the extra time to the "Inside" of the pulses, instead of between them. But, that additional time can be minimized if LX and RX are already scaled to match PULSOUT's 10us resolution (*10).