Short answer: The item says 60 arrows, 18 javelin and 6 weapons...go with it and you will save yourself an headache.
Let's digress...
If you want an answer taken from the books..well I'm sorry but there is not such answer because there are three big problems.
The first is what is a nondimensional space and how it exactly works
It has three distinct portions, each with a nondimensional space allowing it to store far more than would normally be possible.
What is a nondimensional space? Probably we don't know but there is this answer (the accepted one) that can give you a bit more to read and does a wonderful job:
Nondimensional space
Now the second problem is the three containers size:
The first and smallest one can contain up to 60 objects of the same general size and shape as an arrow.
Ok this is simple...we know that this first box from outside is big enough to hold and let enter only arrows so i can not put a bow inside. Cool, no problem here.
The second slightly longer compartment holds up to 18 objects of the same general size and shape as a javelin. The third and longest portion of the case contains as many as 6 objects of the same general size and shape as a bow (spears, staffs, or the like).
Now if a staff can enter an hole...an arrow too can enter that specific hole... and there is not written that the item magically prevent other type of items to enter the quiver if they can fit in.
Finally the third problem:
Once the owner has filled it, the quiver can quickly produce any item she wishes that is within the quiver, as if from a regular quiver or scabbard. [...]
This probably is the worst of the three because is really ambiguous:
- Does it means that i can pull an arrow only from the arrows slot?
or
- Does it means that once I put a bow in the quiver I can pull out that specific bow from any quiver slot?
And the list can go on...
For example the item description uses the word "produce" instead of "retrieve"...it seems that literally the quiver create a new item of the desired type instead of retrieving the item you have put inside.
I think you catch the point.
In these cases where there is not a solid answer and a lot of ambiguity...
keep it simple.
I will not stress this enough probably: simplicity is the key of success in a tabletop game, specially in a complex game like Pathfinder where you already must keep trace of a lot of things.
If the item says 60 arrows, 18 javelin and 6 weapons...go with that even if it is ambiguous and probably does not have a lot of sense.
