Before rP22984 siege units were produced in the fortress and you could house up to 20 siege units there.
Since then, any type of siege unit can only be produced in the workshop. For such a large footprint, it can only garrisson two siege weapons at the time, this patch aims to increase it to five.
The elephant stable also allows for five.
PS: The garrison limit was set to two in rP20457, but without an explanation.