yy.yy.yy.yy というのは何ですか？
ソースを見たところ、[<- yy.yy.yy.yy] というのがあって、それが対象のアドレスと違う場合、その行は無視されるようです。

src/libs/zbxicmpping/icmpping.c の process_ping() 内

/* The were two issues with processing only the fping's final status line:　　*/
/*　1) pinging broadcast addresses could have resulted in responses from　*/
/*　　different hosts, which were counted as the target host responses;　　*/
/*　2) there is a bug in fping (v3.8 at least) where pinging broadcast　　　　*/
/*　　address will result in no individual responses, but the final　　　　　　*/
/*　　status line might contain a bogus value.　　　　　　　　　　　　　　　*/
/* Because of the above issues we must monitor the individual responses　*/
/* and mark the valid ones.　　　　　　　　　　　　　　　　　　　　　　　　*/
if ('[' == *c)
{
　/* Fping appends response source address in format '[<- 10.3.0.10]' */
　/* if it does not match the target address. Ignore such responses. */
　if (NULL != strstr(c + 1, "[<-"))
　　continue;

　/* get the index of individual ping response */
　index = atoi(c + 1);

　if (0 > index || index >= count)
　　continue;

　host->status[index] = 1;

　continue;
}