The value returned by PULSIN for a specific pulse width is dependent of the speed at which the PIC runs.
At 4MHz the resoulution is 10us, at 20MHz it's 2us (all according to the manual).

This means that the PULSIN values corresponding to a logic "1" and "0" will be different depending on oscillator frequency.

I reccon the code you copied comes from Bruce's post #18 in the thread? Is that correct?