Yea, it may be looking little a bit confusing. May be someone else say "why should we care?, both are working and delivering the message to the player !". And in that case i might answer "Yes, you are right". But we study here ourselves, and who we are !
Let me explain more in detail. After checking lots of my partners (Players & programmers) -putting my self in the top of the list- i found it may have a reason inside the developer or the designer himself to write it in a specific way, and this reason reflect lots to his work too. I found the people who write it "WASD", it more easy developer, they take everything easy, and the look for the easy way to fix and solve everything. Or even looks for the easy way to live and do whatever they need to do. Some of them was beginners too !
The "WASD" people-which i will call the easy people- they find it is more easy & quick to write the "A" after "W" and then write the "SD" & it will be a lot more easier for them to do that, specially if they are still new for typing.
The "WSAD" team, most of them was professionals (designers and programmers) and they really are !
They are professionals because they are looking for the most organized way to do the stuff. and trust me any organized person, you can trust ! This type of people prefer to say it like that, because "WS" refers to the same Axis, which will usually control our character movement front and back (you can call it Vertical Axis). And "AD" together because they refer to the same thing too which represented by the character movement of the left and right (we call it Horizontal Axis). So, they put everything in it's right place, even if it will be little complex in writing it or even not easy in the quick typing !
So, is that everything ?
No, hold on. I said this little thing can affect our work. So, how it can affect ?
It make our code easy 7 organized. By default if you are organized person, you will be -according to this quick study- using the second method "WSAD", when you are going to define your inputs inside your code, or you are going to write an input class from scratch, you will write the horizontals and then verticals or vise versa. And this will make your code looks more organized for yourself or even for someone else who will be able to check the code after you.
But if you are from the first team "WASD", you will define the player movement on a single vertical axis, then single horizontal axis & then repeat it again. and it will looks little messy !
Yea, what matters, that the player will use the four keys in all this cases, and if you write to him "WSAD" or "WASD" he will get the idea of how to move the character. But here am talking about ourselves, who we are & what we are doing ! All what matters for a coder it make the best code possible to be able to recycle and use again. All what matters for a game designer it make everything clear. So, what do you think ? in in which team you are ?