It's simple. Raises aren't there to reward you for what you've done. They are they to keep you working at the same company. Meeting a goal of what you did last year is inconsequential to the business trying to keep you. You got paid last year to do what you did. The raise is to convince you to stay and do it again the next year. The business has to decide if you are worth investing in to further its objectives.

Here's an easy example. If you were going to get rid of someone in 4 months after their review, would you give them a raise? Of course not, you'd allocate that funds to someone else.

That's why it's hard. The business has to judge which people they are going to focus on keeping.