There is more than one Saint Iosaphat. There are seven Iosapaphat mentioned in the bible, so there are seven St Ioaspaphat recognised and celebrated by the Greek Orthodox tradition (alone). So the saint in question (the Rusian saint) may not necessarily be named after the specific St. Iosaphat Son of the Indian King Avenir. This is why there is a epithet attached to the name, to distinguish which Iosaphat is pictured in the icon and is being celebrated.
For example I am named Gregory, right? I am named after Saint Gregory the Theologian of Cappadocia (Nazianzou, to be exact), he is celebrated on the 25th of January according to the Greek Orthodox tradition. But there are another 40 Saint Gregory recognised by the Orthodox tradition and celebrated on different days.