This is because when stored in EEPROM the water time is converted to 1-byte representation. It uses a non-linear scaling method to do this conversion. Because OpenSprinkler supports a large number of stations (56 in the current firmware) and a large number of programs, in order to fit everything in the limited EEPROM, it’s necessary to reduce the storage size for water times.
The relevant discussion is in this thread:
https://opensprinkler.com/forums/topic/survey-can-station-water-time-fit-in-1-byte/
The current compression scheme is based on the fact that very few users need second-level granularity at all. Most users need just minute-level granularity. In addition, this only affects the stored water time — at run-time the actual water time is the stored water time multiplied by the watering percentage, and it is down to resolution of seconds.
The API is designed to be transparent of how the water times are stored internally, so when using the API you don’t need to be aware of the compression method.