nSequence and opt-in ReplaceByFee: difference between maxint and maxint-1?

If I understood correctly, an nSequence below 4294967294 (or 0xFFFFFFFE in hex) indicates the transaction as opt-in RBF. However there are many transactions with nSequence 4294967294 or 4294967295 (0xFFFFFFFE or 0xFFFFFFFF respectively). What's the meaning of these two nSequence values? I thought 0xFFFFFFFF (i.e. max uint) used to be the default for regular transactions. Why are there also many txs with 0xFFFFFFFE (i.e. max uint -1) ?When I look at recent transactions, e.g. unconfirmed ones in the mempool, or txs that were confirmed in recent blocks, I'd say about 50% has nSequence=0xFFFFFFFE and 50% has nSequence=0xFFFFFFFF. However for older transactions (like 1 year or even further back) practically 100% of all txs have nSequence=0xFFFFFFFF. What's up with that?Or am I mistaken, and does nSequence=0xFFFFFFFE (rather than below 0xFFFFFFFE) represent the opt-in RBF flag? via /r/Bitcoin http://bit.ly/1QDkF3V