The definition of "about" is
Used to indicate movement in an area
While "in" is
Expressing the situation of something that is or appears to be enclosed or surrounded by something else
So, here, "about" is more specific. "She kept walking about the room." brings up a mental image of a woman pacing around a room. "She kept walking in the room" brings up a mental image of a woman walking in a straight line in a room (to me at least) Aside from this, "she kept walking in the room" has another meaning-- "she kept entering the room", which is a completely different situation.