I posted this explanation on another thread but I think it addresses what you said:
I think that LE has the information that they say they have. It's the information that they aren't saying they have that is important. They are being strategic with the info they release. What I'm unsure of is if they ALSO have information that disproves their evidence. Ok so you have Caylee's dna in the trunk and you release that information. So perception sways one way. What if they have other's dna in the trunk as well? They haven't released that info. That would lessen the impact of the info they released about Caylee's dna, right? So they won't do that, imo.
They are trying to get something from Casey so the information they release isn't really about pleasing the public and being 100% forthcoming, it's about getting Casey to tell the truth by any means and within the constraints of the law. Only releasing evidence that tells one particular story is a way to do that, IMO.