There are typically multiple ways to write code to address a given issue. They might differ in aesthetics (including clarity of intend), but also performance, maintainability, extensibility and robustness. It's not uncommon that those who wrote the code to be fixed rewrite the patch in order to improve on aforementioned properties while avoiding lengthy back-and-forth when asking for improvement of the patch. They are often in a better position to do so due to familiarity with the code, it's history and (unwritten) development path.

That is by no means plagiarism. If not communicated clearly, can however lead to disappointment of those who submitted the patch. An 'inspired-by' comment would have been nice.