The question comes up on message boards and the like: what scale does 15 mm equate to - 1:100? 1:110? And the answer inevitably comes back, well, the correlation is not exact. This is because the “scale” expressed in terms of millimeters is neither standard from manufacturer to manufacturer nor a true “scale.” The most common stated definition is that a standing man measures 15 mm from the sole of his boot to his eye. (This measurement is used because so many soldiers wear elaborate headgear, so a measurement to the top of the head is not possible.)
One of the immediate difficulties is that the height of the average soldier has not held constant over time. Men have been growing significantly taller over time. While not an issue for most of the soldiers, it does mean a Napoleonic officer should be a taller figure than, say, a Roman centurion. But if every army follows the definition above, this will not be the case. Of course, no harm there, since these two figures would not be on the same table at the same time. But if some manufacturers account for this height change and some do not, it means there will be variance from manufacturer to manufacturer.
A second immediate issue is what period to use as the benchmark? Should the modern soldier at 5’ 11” be 15 mm and prior periods adjusted accordingly? Or make an ancient Greek 15 mm and adjust modern soldiers up? (The Miniatures Page has a tool that will convert height in millimeters to scale for you.)
The larger issue seems to be that this “scale” is treated as more of a suggested guideline. Hence even the renowned sculptor Anthony Barton suffers from what is called “millimeter creep.” This is the tendency for some sculptors or manufacturers to gradually increase their size over time. When this happens you get some size distortions even within historical periods. His latest Napoleonic models, for example, are nearly 20 mm and hence look like giants on his 15 mm horses.
Sculpting quality and aesthetics aside, it is this issue of scale that drives the incompatibility of many miniatures lines. Two miniatures from different manufacturers may look freakishly distorted when seen side-by-side. Hence most gamers will make up units from just one manufacturer. If they end up with figures from multiple manufacturers, they will be in separate units. But as can be seen from my British group photos, even at a small distance, on the gaming table the differences will not be all that noticeable.
In the end, you’ll simply have to pick the manufacturers you like and live with any of the resulting issues. Still, if all the gamers out there can push for change, perhaps we can do away with the millimeter designations and move to true scales.