It is a ploy that some schools decided to do to "trick" some signees into thinking they are signing a bindable agreement when in reality, they aren't.
Based on the number of initials a school may have and the differences between signing an NLI and a grant-in-aid, some schools have "tricked" signees into signing something during the early signing period. That way, those signees think they have fully signed-on and can't change their minds or be recruited away by other schools.Those signees will have a big presentation at their school, post pictures of them signing on social media, etc. In reality, they aren't signing what most of the others are. They won't be able to sign a bindable offer until later. It mitigates the chances of losing that signee.
Other schools, assuming the recruit has signed somewhere, tend to back off which leaves him less susceptible to being poached by another school . . . unless of case the signee discovers that what he signed isn't binding.
So, that leaves the question of why some schools didn't announce their class from the early signing period? Well, assume you had 10 guys sign binding agreements and another 4 sign non-binding agreements. Schools can only announce those who sign legitimate, binding agreements. If the school announced those 10 signees and not the other 4 who thought they had also signed a binding agreement, the school would have some explaining to do . . . and there is no easy way to do that without offending the recruit, pissing him off, show him that you weren't necessarily up front with everything, possibly lose him to another school.
It is a good strategy some schools have selected to do, albeit one rooted in some shaky ethics.