If you are paid 100 for 10 hours then pro rata you would be paid 50 for five hours and 10 for one hour.
If your working week is 40 hours and you reduced your work to 31 hours you would divide 40 into your salary and multiply by 31 to get the pro rata salary. Your hourly rate would NOT (should not) change. The same applies to prices or offers to supply goods. Hope this helps.
If a full time job at a Company is 40 hours per week for �20K and you see one advertised at the same Company for 20 hours for �20K pro rata, you are working half the hours so get half the pay - �10K.
It just means in proportion but I can't see why employers can't just say what the employee will actually get since plenty folk are not sure what pro rata means.
Sometimes the salary is fixed for a post in negotiations between unions and employers, especially in public service.
European legislation then provides fo ra range of "family friendly" working options whereby, especially in public sector, workers can seek to work for less hours. The conditions and terms insert the pro rata clause to ensure there is clarity for both employer and employee. Better to see it there than not, I would believe.