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.
Wasn’t this also the case in medieval England? The church rang the hours, but the time between them was calculated based on the length of the day?
The fixed-length hour is a surprisingly modern conceit, like universal timezones.