as mentioned above, sperm carrying the X chromosone (female) tend to live longer then the sperm carrying the Y chromosone (male)
my only assumption to this myth is once u go on cycle ur body stops producing its own sperm
therefore at some point as you begin to produce less sperm and eventually probably none, the male sperms die quicker and the female sperms are still present for a greater period of time
thus you are left with a greater ratio of X/Y chromosones for a period
this increasing ur chances of having a girl for a period of time while on cycle (until of course ur on long enough where ur most liklely shooting blanks coz all ur sperms died and u aint producing shit)
of course this is theorectical, but its the only explanation i can give to this myth