Roman hours were not of a fixed length because they simply equal to the amount of light or darkness on a given day divided by twelve. Since the amount of daylight varies greatly from day to day over the course of the year—with perhaps as many as 15 hours of daylight in the summer and only 8 or 9 in the winter—a Roman hour in the summer might be equivalent to a modern hour and a half, and, similarly, in the winter, a Roman hour might be only 40 of our minutes long.
That’s almost a necessity if you use a sun clock as your primary time measurement device.
How so?
Before the sundial, the day was just before noon and after noon. The 12 hour day (and 12 hour night) came in once they had sundials, 263 BC.
Sundials do not need calibration for latitude or day length. Romans made a conscious choice to have 12 day and 12 night hours rather than sunrise and sunset at different times throughout the year.
See https://www.macmillanhunter.co.uk/time/measurement-of-time/what-is-the-equation-of-time/ for an explanation, and how it’s corrected on sun dials.
That’s super cool, but it’s just a distortion of time on a sundial, it’s not a purposeful distortion of hours. Nor does it align with the summer and winter hours (the distortions are offset from the solstices).