Difference of the following two Fully Differential Amplifier
'I have checked both data sheets and haven't found anything different'

Well you haven't checked very carefully , have you?

They seem to be very similar. There is at least one difference, the bias current of the 'A' version is higher than the plain version.

Given that the plain data sheet is dated 2012, and the A version is 2016, I suspect that what has happened is this. They wanted to discontinue the old process that was used to make the plain version, and move the part to a newer, faster, finer process. They would have needed to change the masks to fit the new process, and either couldn't match the previous bias current, or couldn't be bothered to spend the time to, once they had matched all the more important dynamic, gain, distortion, noise, drive current etc etc specifications.

As the new part could fail in some existing circuits, if the customer had designed it right to the very edge on bias current, they gave it a new part number.

I would be willing to bet that the new part does not match the old part in all the parameters it appears to, but is actually better, probably faster, due to the new finer geometry process. But once the new part does better than the old one, there is no real need to change the data sheet. Only for spec degradations is there a possibility of upsetting existing customers.

There is another possibility. Their old process has 'gone bad', and try as they might, they just can't meet the original specifications. This happened with their low noise OPA656. The original device has been in the field for so long meeting specification that they can't just degrade the spec and hope no-one would notice, so it needs a new part number.