Your project quota and voiceover quota serve distinct purposes. Your project quota is related to video downloads, and it is only counted when you download a project. On the other hand, your voiceover quota is counted each time you apply an AI voice to your project.
It's worth noting that, only Premium voices, which are powered by Eleven Labs have a quota, while Standard voices, do not have a quota restriction.
In essence, you can use up your voiceover quota without touching your video quota, and vice versa.
If you're curious about how your voiceover quota is calculated, you can find more information on that here.