Eastern Astrology present in VMS

[ReneZ]

I would rather say that they are equivalent in all positions, but certain shapes are preferred in some positions. 
Perhaps the (hypothetical) draft that the scribes were copying from was not very clear.

I have the same feeling about o and a. Since you have looked closely at the handwriting, you will have seen that o and a are frequently hard to distinguish, almost as if it didn't matter which one to use.