I think what happens is that each player can only receive 20 maps per day. So If they send a request out to say 100 neighbors... after the first 20 respond the requests go away. After a day goes by... then the requests reappear in their neighbors messages.
I have sent out a request .... got my needed amount to expand... then over the next two days will see the post that a neighbor responded to my request for maps ... even 1-2 days after I sent out the request... so I think they linger in limbo for the next few days.
I hope I explained it how I understand and I hope this helps you better understand... but yes... it is normal to respond to all your requests and then a little while later see ones from several hours or even days before..