WHY a UK (PS) vs. US (SS) edition and other newbie musings

[Darlene]

No: HPFGUIDX 30672

So, just the other day I received my very own copies of the UK 
editions of the series.  I have read the US editions twice 
(borrowed.. guess I'll have to buy my own copies!).  I'm wondering, 
WHY are there two editions?  Did the US publisher think we would not 
be interested in reading the UK version?  Is this a common thing to 
do?  
Why did Chris Columbus then make two versions of the movie.  Seems 
silly to me.  I'm sure I would have enjoyed it just as much if they 
said "Philosopher's" instead of "Sorcerer's".  

Also, why, in the US edition, are things changed?  To the point of 
being errors?  For instance, after Harry is sorted, the US version 
says there are 3 students remaining, when in fact there are four (the 
addition of Dean)?

Perhaps... they know that suckers like me will be looking to buy the 
US and UK versions... hmmm...  they could make a killing when this 
thing comes out on DVD.  Amazon.uk here I come!

Dar