Aspect Ratio describes the proportional relationship between the width of an image and its height.
A 6 x 4 picture has an aspect ratio of 3:2
Most point-and-shoot cameras and older televisions sets have an aspect ratio of 4:3
A picture taken with a 4:3 aspect ratio will not fit on a 6 x 4 piece of photo paper.
To make it fit, you have to either adjust the size of the image or cut-off the top and/or bottom of the image.
So photo printer services do one and some do the other.
If some photo printer services are not consistent, only they can tell you why. Ask them.
All about aspect ratio:
http://reviews.cnet.com/aspect-ratio-guide/
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I often upload pictures from my digital camera to Walmart, Snapfish, Shutterfly etc. When I have finished I often get a message that says something like "some of your pictures were truely digital" or whatever, and that I should choose something other than 4X6 etc. usually like 4X5.5 etc. I say OK and I get the pictures, and some are 4x5.5 etc. My question is...all the pictues were taken with the same camera, why is there a probem with always a few ??

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