Prime formatting behavior

[Camille]

Hi,
I have version 2013 8 13 (5106) of the Prime, and I see the following behavior in CAS mode. I would like to understand better what it means:

I enter "diff(cos(x))" and see
diff(COS(x)) -> -SIN(x)
[sounds good]
Then, I press [UP][UP][ENTER] to retrieve my input, and it copies the following into the entry line:
cos(x)'
ENTERing again shows again
diff(COS(x)) -> -SIN(x)

What is the reason for switching between low caps in the entry line to big caps when displaying the history?
Similarly, what is the reason for changing the differentiation notation?