Do students needs to be specifically mentioned on grant proposals for projects funded by government agencies in the US?
My experience is that students are not put in NSF proposals by name, but a slot is put in the narrative and budget with the name listed as TBD, "To Be Determined". It's possible that the PI failed to count all the heads correctly and was off by one. I would be surprised for this to happen in a small group, but if there were 10-12 students in the group, it could be easy to miss one.

Edited to add: It's worth noting that a particularly excellent student with lead or primary authorship on the papers that constitute the prior work for the grant might be highlighted in the grant text and budget, especially if the student has won some awards for the work