doubleofive said:

I always assumed that (a) it was her first scene and she thought she should have a British accent, which they later changed (you'd think they'd fix that in post) or (b) she was in such awe of Peter Cushing that she found herself speaking with his accent.

I took it as something she adopted when speaking in a diplomatic/official capacity, reverting to her normal speech patterns in casual conversation. This seems reinforced by the prequels, where Padme adopts different tones in different scenarios.

Or maybe the director wasn't so good at helping actors with their characterizations. Could go either way.

Ron Moses