我见过一些在decision后面有后置定语,可前面的冠词还用a的情况.比如：a decision unfavorable

我见过一些在decision后面有后置定语,可前面的冠词还用a的情况.比如：a decision unfavorable to the opposition.He has made a decision that he will be an inventor some time.
照理说,定语表示特指,前面应该用定冠词the,可这里用的是a,是因为make a decision是固定用法吗?
如果make the decision 可以的话，a decision unfavorable to the opposition中为什么要用a？