The december entry is clearly Olga's. She did the beds, as usual: that was Olga's work at her lazaret, not Anastasia's who only visited the wounded. Then, "played... with Kassianov". Kassianov was a wounded officer of Her Majesty's lazaret. He played violin and Olga had a frendship with him, as we can guess.
About the 12.08 entry. I think (that is only my opinion), that is came not from Anastasia's diary but from an Anastasia's letter to her father. It is her style, she described things we can believe she, and not O or T did.
An other mistake in the book Sisters of mercy:
entry of the 18 october 1916: it is clearly not from an Anastasia's letter but from Olga's diary.
Same thing the 11.12
Not a letter of Anastasia, but Maria's diary entry.
The 13 and 14 june 1916
Diary of Olga, not Tatiana
22.07
Diary of Maria, not Tatiana