Maybe her mother had designs on Colonel Brandon but then Mr. Palmer came along who was richer (I guess?) and actually showed an interest in her daughter and she focused on him instead. That would be consistent with Mrs. Jennings character. It would also be consistent for Mrs. Palmer to twist it around and say/believe her mother wouldn’t allow it. (Much like when she says Brandon told her Marianne was engaged but then when she repeats the conversation he said no such thing.)