請幫我看一下USCIS的回複，謝謝！

上個月發了一個I485的service request，今天收到了回信：

Based on your request we researched the status of this case. We are actively processing this case. However, we have to perform additional review on this case and this has caused a longer processing time. If you do not receive a decision or other notice of action from us within 6 months of this letter, please call customer service at the number provided below.

想請問一下在什麽情況下，會需要additional review？多謝！