35=2 after 35=4 failed because "MsgSeqNum too low"

This scenario happened in a poor network environment where some messages are missing but heartbeat still working, no logout before issue happened:

1. My app is client side using quickfixj-all-1.5.3.jar.
2. When network has problem, some messages are missing (both sides)
3. Sometimes server side send 35=4 request to increase seq_no by 1 while there's no missing seq_no from server side. E.g.
34=100, 35=8......
34=101, 35=8......
34=102, 35=4, 36=103, 123=Y...
I don't know why server side send this 35=4 asking to reset seq_no to 103 (seq_no 100 and 101 received successfully and 102 is reset request itself)
4. If next message from server side is 34=103, 35=8, it's OK
if next message from server side is 35=103, 35=2, client side FIX engine will return "MsgSeqNum too low, expecting 103 but received 102"

The question is
1) why 35=2 after 35=4 fail while 35=8 after 35=4 OK
2) the seq_no of failed 35=4 after 35=2, I double checked it's 103, why response is "MsgSeqNum too low, expecting 103 but received 102"