[throwaway7033]

Job descriptions are forward looking statements; the position may be entirely different.

Here are some of the many possibilities:

The service has poor uptime and you're on call every weekend.

Schedule pressure or poor management means that you're asked to come in on Fridays "until the next release".

The reduced-hours position was sponsored by a VP who has lost interest or left the company. Your direct manager has deadlines to meet and pressures you into working full-time.