And it’s nothing to do with when it was made or what material they used for the tesserae (Latin for mosaic tiles).
To understand this we first need to look at examples of their architecture, their temples are good examples of this. Copying the Greek examples they used certain geometry to not just ensure the buildings would stand but, they wanted them to be aesthetically pleasing and within certain proportions. Wherever they were in the empire these structures were built using the same principles.
With the same objective in mind and, following the mosaic work of the Greeks, they also ensured their floor mosaics were constructed in such a way as to create mosaics that were organic, balanced and aesthetically pleasing. They achieved this by using a set of eight rules that all mosaicists used wherever they were in the Roman world.
While not as precise in the way that geometry is applied in construction and design and taking into account differing standards of work they nevertheless had these rules used from Syria to Brittania.
The easiest of these rules to see is the borderline rule. To see this look at any figure in a mosaic and notice there is a line of tesserae (the same colour as the background) running around the figure. Once you start to notice them then you will see they are in all the mosaics and it is not just around figures but in some patterns too. This specific setting of the tesserae has been referred to as andamento but it is a very specific type which is why I use the term ‘rules’ to describe them.
You can see in the photo above how there is a line of white tesserae around the dolphin and then the background is filled in. What does this line do? Look at the drawing and I will explain more.
At the top half of this drawing, you have an area of the mosaic where there are different colours. Where these colours meet on the diagonal section then you have to cut triangle shapes on either side to ensure there are no big gaps in the mosaic. This line of triangles of different colours creates something that is artistically ‘sharp’.
Where the colours change with a horizontal line between them then obviously there are square tesserae meeting square tesserae. This creates a ‘soft’ line. With these two together you have an area of the mosaic where, visually speaking the lines appears alternating between sharp and soft, sharp and soft. This creates a discordant effect that could draw in the viewer's eye when you don’t want any one area of the mosaic to do.
By adding in the borderline of each colour tesserae it has the effect of pushing the triangle shapes away from each other and into the background so they are not as noticeable.
In the same way as the geometry used in temples and other important buildings, attention is paid to the visual impact. Whilst this does vary considerably between the different mosaics due to the mosaicists and workshops having different standards of work, these rules are always there.
For more publications and information on making copies of ancient mosaics, go here, https://www.ecwid.com/store/roman-mosaic-workshops-/
To learn more on video see my Youtube playlist here, https://www.youtube.com/playlist?list=PLku2S5ge3WLwa4tbrgKyzCsb_DFmoHD_7