Laser diffraction measures the dispersed state of a sample. That simply means that the laser diffraction instrument measures the particles as it is presented to the optics. If the particles stick together (agglomerate), that is the size that will be measured.
So if your sample tends to agglomerate slightly how you cope with that agglomeration will influence the result you generate.
Instrument settings also play a role. Factors like refractive index, adsorbtion index, distribution model and physical settings like pump and stirrer speed will all have an impact on your final result. That is why method development is so important when you measure a sample for the first time. During the method development you will evaluate the impact of these settings and select the appropriate settings for your sample material.
That is also why we spend the bulk of our time on method development when we train new users of our laser diffraction instruments.
It is therefore clear that if you want to compare results between instruments or labs that all the instrument settings must be clearly defined and agreed before you start testing!
Representative sampling should also be kept in mind, especially if the size distribution is larger than 100 um. For these types of samples spinning riffling is normally recommended to create appropriate sub samples.
The advantage of laser diffraction is that it is a relatively quick technique (typical sample to sample time is less than 5 min) which means you can investigate all the factors mentioned above relatively quickly.