Tom is apparently British - that's how the time is said in Queen's English.
Listen to the BBC sometime "The time is now eight hours, GMT."
Edit: Nope. You have your Language & Region setting set to "Time format: 24-Hour Time". That's how it pronounces it when you have that setting set. Note: *NOT* the "display time in 24 hour mode" in the Date & Time preference pane, but the Time Format setting in Language & Region.
Uncheck that box, and it will use "o'clock" like you expect, for every voice.