Can my employer limit my vacation accrual for a year based on a carryover amount?

Based on my time with the company I should accrue 200 hours (5 weeks) of vacation a year. The company has a written policy that if you do not use all of your vacation at the end of the year you are allowed to carry over 120 hours- anything else will be forfeited. However, I have been informed that basically if I carry over 120 hours then I will only accrue 80 hours for the year if I don't take any time. They are saying that I can't accrue more than 200 hours a year. This doesn't seem right. I don't necessarily have heartburn with only being allowed to carry over 120 hours but if I work the hours to accrue 200 hours I should actually accrue 200 hours. There is nothing in writing that indicates you may accrue no more than 200 hours.