yes, it is strange. but some players for some time don't seem to be able to beat others, even though the latter may be defeated by lesser players:
martina couldn't beat lindsay 1999-early 2000
martina can't beat jennifer 2001-now, but williams, esp serena, could
monica couldn't beat players ranked ahead of her since 1997 until recently when she beat martina twice (end 2001) and venus (2002 aus open)