I think I know the answer to this one. Syria starts as part of the Mamluks so I suspect that it broke off in a "War of Independance." If so, it could have annexed Mamluks in one go.
Let me explain this in the context of game mechanics: When a country declares independance, the rebel state might either be large or small, but the overlord must have the same possibility to reassert control over the separatists. If the rebel state is small, like an OPM Orléans, there is no problem applying the regular annexation mechanics with a 0 BB cost per province. However, if the rebel state would be very large, like a 20 province France, the province value would have been too high to annex the rebel state if the value limit was applied. To be able to defeat large separatist states in Independance Wars, the regular limit has been removed for full annexations, as long as you occupy the whole state. The alternative would be an automatic fail if you allowed to many rebels to defect at the same time, since you would have to make an incomplete peace with the rebels.
This, however, can create some pretty hilerious results for a very simple reason, namely that most CBs in eu3 work both ways. If someone declares war on you with a Holy War CB, you will also be able to claim province using the same BB reduction and mechanics. One's crusade is the other's jihad, I guess. The same goes for Independence wars, something that was perhaps not entirely intended. This means that while the overlord can annex any rebel state, the rebel state can also annex the whole of the mother country BB-free, if it occupies 100 % of the provinces. Usually, the rebel state is smaller/weaker than the mother country, which means that it will either lose or sue for a WP. When the mother countries has been weakened by external wars, however, it is entirely possible for the rebel state to annex the whole of the mother country. Since Syria usually appears in the game after a successful Independance War with the Mamluks, I suspect that this was precisely what happened.
I've seen this happen to Sweden after Finland breaks off on a few occations, but there is perhaps an even more interesting example. In the "American War of Independance"-scenario, it is actually possible for the US to annex the whole of Great Britain in the unlikely event that you occupy every province in their global Empire -- unless they've changed things in patch 5.2
