Compensation and Overtime: Part Time Pay

My question involves employment and labor law for the state of: WA
I work in a position classified as non-exempt. We have both full time and part time employees. I work part time and am compensated with an hourly wage. However the full time employees in the same job are paid a set, monthly salary, regardless of hours and workdays in each month. I also accrue sick leave, annual leave time. A full time employee will earn 8 hours of annual leave per month, a part time will earn a pecentage. The issue I have is with the calculation of the percentage. We are hired and given our appointment letters indicating our percentage. Mine is 50% a half time position and I work 20 hours each week. I work 2 1/2 days per week. This is where the issues arrise. My leave benefits are not based on my 50% they are actually calculated each month based on the # of hours I was in paid status as compared to the number of hours available in the given month. Given I work 2.5 days, and have two days off, there are some months were looking back over a month, I have less than 50% of the hours, though I do work exactly 20 hours each week. The contract language is somewhat ambiguous and I worked through the grievance process, but to no avail. There are several months were in order for me to get the 50% leave accruals and holdiay pay I have to come in to work additional hours, in one month it can be as much as 16 additional hours. However, we do have full time employees who work 4 10's. If a holiday falls on a day they are scheduled to work 10 hours, they see 10 hours compensation for that holiday. I work 8 hours on Monday, but if that holiday is on a Monday, I receive only 1/2, that's fair, but I must come in an additional 4 somewhere else in that month to have my accruals stay at the 50%. If this same logic was applied to full time that person with the 4 10's should only receive 8 hours on that holiday and have to work an additional 2 in that month.
Bottom line, can you calculate employee's pay this different? For a year, they did pay part time employees salaried, at their percentage. I loved that, there was never an issue and my paycheck was consistent. We are paid twice a month, not biweekly.