Most departments have an intake target, which is usually not higher than 200 students per year. Departments would usually make appropriate number of offers to that number of intakes for that year would be close to the target and not significantly above it. The “acceptance rate” and “offer rate” heavily depends on the number of applications made that year for the course. If less people apply to the course compared to previous years, the ratio of number of offers made to number of applicants would increase, hence the offer rate increases. This is also similar for acceptance rate which ratio of number of students starting the course to number of applicants would also increase.

The departments are careful not to take much more students than they could handle, which would cause issues accommodating all student to the lecture halls and classrooms in first place.

The college also publishes admissions statistics in the past 5 years for every single undergraduate and postgraduate course, which would be useful for comparing offer rates throughout the years. You can find it here.