The pay raise was probably for additional responsibilities. I recently had the manager of my lab leave for a better job. His #2 took all his responsibilities, so she was promoted to officer with a pay raise, not manager. This may be because you can do the job now, but you may not be as skilled a worker as they require for management. I have already decided that she will be my next lab manager, but am waiting to promote her for two reasons:

1) To see if she can manage her staff well and set up a successor as her predecessor did.

2) To let her gain some experience before she is promoted to manager. This means she has access to all the funding she needs from the company to educate herself and take courses, leadership classes, etc. If she can manage, train, and lead the team to success she will be the next manager.

They are most likely gauging your performance and will make the decision to promote you when the time is right. Don't set your mind on a promotion though, as they can still choose to bring someone in from outside the company to fill the vacant position, in which case you will have a new boss. But congrats on the pay raise, was probably well deserved!