You may be referring to sieving or laser diffraction, but since the latter is more interesting, let’s focus on that. We must first assume that both the dry and wet accessories are performing to the manufacturer’s specifications. If the material is in the same state of dispersion, then the result should be the same whether wet or dry - all other things being equal. So the first possible reason is inadequate dispersion in either the wet or dry accessory, but we do need to take a step back, as the measuring device can only be as good as the sample given to it, bringing sampling into question.

Let us assume that we are dealing with identical subsamples of the same material, or at least have allowed for this in the comparison. Typically, the sampling issue raises its ugly head when material greater than 75 or 100 µm is present in a system. If in doubt, divide the powder on a spinning riffler or using a liquid sampler (called a Burt sampler). For coarser material, it is important to use a wet disperser that has a larger capacity to ensure representative sampling. If there is material smaller than around 25µm present in a system, we look at dispersion. In order to look at dispersion we have to examine the effect of input energy on (reported) size. In our wet methodology we should sonicate to stability, and in our dry analysis we conduct what is termed a pressure-size titration, where we look for stable regions in the distribution.

Earlier I stated ‘all things being equal’. Some things that may not be equal:

Optical Model choice: With dry dispersion we know the medium is air, with an Ri of 1. With wet dispersion, it is important to choose the right value for the medium liquid.

Dry measurements made with excessive air pressure may cause the material to break-up or comminute

Choosing the wrong liquid medium could cause dissolving, swelling, or a reaction. A microscope next to the dispersion unit and removal of a drop of material at the appropriate stage may be a very useful guide
here.

Cleanliness of the disperser and optical surfaces (are windows stained or scratched?)

For additional references, I suggest you consult ISO13320, the international standard for laser diffraction, and Terry Allen’s book “Particle Size Measurement” (now in its 5th Edition).