Re,

That's simple :

The channel signals follow each others within ~ 10-15 ns ( nano seconds ! ) delay ...

PULSIN need a little time ( some µs ) to be ready to catch the signal. (see PbP libs ! )

those µs being more than the ns channel separation time, the second PULSIN doesn't see the second pulse rising edge ... and must wait for the following frame.

That's all

Alain