Perhaps that's the central theme of the film (and it appears somewhat likely) - that Tyrell Corporation has always had the ability to make replicants who have a normal human lifespan. Deckard could be one.
Replicants as I understand are not required to have a limited lifespan, this was put in place as they found that if a replicant is alive for too long they eventually revolt.
If you go by the theatrical cut, Rachel did not have a limited lifespan. This makes sense since Rachel had memories implanted into her and it's established that she doesn't know she's a replicant. Presumably, if Deckard is indeed a replicant, it's likely he's the same as he doesn't know he's a replicant either.