Before the fork on Oct 31 there was one blockchain, SIA.
After the fork happened the blockchain split into two just like a fork does (in fact, there were more fork teeth but let's forget about this for the sake of simplicity).
Before Oct 31: one blockchain, "SIA" = "SIA Classic", all blocks and all transactions are identical.
After Oct 31: two blockchains with different blocks and different transactions. You could choose either of them. In fact, you could even choose both. The blockchain you're on depends on the client you use.
From Siamining perspective, the blockchain you mine on depends on the port you are connected to and the hardware you use (Obelisk/Non-Obelisk).
This is why you can see your pre-Oct-31 SIA transactions with your SIA Classic wallet.
This is probably why you can see your post-Oct-31 transactions with the outdated SIA wallet. It looks like you're on SIA Classic indeed.
I'm not sure if the wallet address can stay the same both for SIA and for SIA Classic. But the logic above assumes that it can.