If you went to Amazon, go back there and read the reviews. You will see there is a controversy as to which order they should be read in...the publishing order or the chronological order.
This should indicate to you that the order in which they were first published, the order in which Lewis wrote them, was NOT the order of the set you have. The publishers decided to re-arrange them.
If you haven't read the books recently, go back and read The Magician's Nephew. You will see that when Lewis gets to the creation of Narnia, he is obviously explaining something that he assumes the reader already has read.
Magician's Nephew is book #1 because it goes into details of the creation of Narnia, much like the Boook of Genesis of the Bible goes into details of the creation of the Heavens and the Earth, among other things.
This should indicate to you that the order in which they were first published, the order in which Lewis wrote them, was NOT the order of the set you have. The publishers decided to re-arrange them.
The publishing date means nothing. Lewis admitted he didn't write them in order. Years after he died, the publishers rearranged them in the proper order. There is a reason why there are book numbers on the spine of the book. If you read them in numerical sequence, you can go through Narnian history, much like reading from Genesis (which talks about creation) to Revelations (which reveals things to come in history, including the "Final Battle" aka Armageddon) in the Bible.