Yes, I had a similar question. That is, what kind of cemetery was this?
Perhaps a non-Jewish cemetery would have allowed Jewish markings, but only if a cross also appeared or maybe this was military? I do remember once seeing a completely Jewish stone in a Catholic cemetery. Its a bit of a long story---but just to show that odd things pop up from time to time.
More likely, the family just wanted to accommodate the decedent's wishes, but also keep something of the family heritage.
Also, did the family legend predate anyone having seen this stone? And finally, from what time period is the stone?