Today in English, a hundred is always taken to be equal to 100. However, before the 18th century, it could mean other values, depending on the objects being counted. Sometimes the value of 100 was referred to as a small hundred and the larger value of 120 was referred to as a long hundred. This larger value originated with the various Germanic tribes that invaded England after the Romans departed.
The word "hundred" followed by "hours" is used in a special way. For example, "eighteen hundred hours" can mean a period of 1800 hours (75 days), but more often it is military time terminology for the time 18:00.